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Lab manual for GEOG1120 MINERALS To introduce minerals and rocks, we must understand a little bit of chemistry. The terms ELEMENT, ATOM, and ION will not be discussed here. But they must be understood, so look them up. CRYSTALLINE SUBSTANCE refers to a solid material that has a definite atomic structure and a definite chemical composition or formula. ATOMIC STRUCTURE or CRYSTAL LATTICE refers to the regular, ordered arrangement of atoms or ions that make up a crystalline substance. This arrangement is the same for every specimen of a specific substance. CHEMICAL FORMULA indicates elemental content of composition. For a given crystalline substance, the formula is either constant or ranges within definite limits. Note that each crystalline substance has its own particular formula and structure; its formula indicates what kinds and what proportions of ions are present, and its structure determines how ions are arranged in space. MINERALS are naturally occurring, inorganic, crystalline substances. ROCKS are naturally occurring, coherent aggregates of minerals. CLASSIFICATION OF MINERALS Although minerals have been studies for hundreds of years it was not until this century that advances in chemical analysis and the discovery of x-rays made it possible for us to understand and appreciated the order that exists in the mineral kingdom. The details of these studies are too involved to cover in an introductory course such as this; however, it is important to know that minerals are classified (grouped) on the basis of the kinds of anion (negatively-charged atoms or atom groups) in their structures. Many of the characteristics of minerals within a class are similar because their chemistries are similar. A good example would be the carbonate minerals. All of these minerals will effervesce to some degree with acids, giving off carbon dioxide gas. The major groups of related minerals are called classes and are named as follows: MINERAL CLASS ANION EXAMPLES Native elements no anions; minerals consist of one element only sulphur, graphite, diamond, native copper Sulphides sulphur (S2-) pyrite, galena, chalcopyrite, sphalerite 1 Lab manual for GEOG1120 Oxides/hydroxides oxygen (O2-) and/or Hydroxyl (OH-) limonite, hematite, corundum Halides fluorine (F-) or chlorine (Cl-) fluorite, halite, sylvite Carbonates carbonate (CO32-) dolomite, calcite, siderite, azurite Sulphates sulphate (SO42-) gypsum, anhydrite, barite Phosphates phosphate (PO43-) apatite Silicates silicon-oxygen tetrahedral (SiO44-) olivine, quartz, in a number of different linkages feldspars, micas, amphiboles, pyroxenes, garnet, clay minerals, talc In nature about one third of the known mineral species (about 2500 to 3000 minerals) are silicates, and they make up about 95% of all the minerals in the earth’s crust. The remaining 5% include all the other classes of minerals, which are termed as a group, nonsilicates. A mineral is defined as a chemical element or compound that is a naturally occurring crystalline solid and is formed as a result of inorganic processes. Each mineral has characteristic physical properties that are easily determined and are therefore useful in mineral identification. It will be important that you learn how to recognize the different types of physical properties and identity the mineral from these characteristics. Your knowledge of minerals will be tested with a lab test later in the semester; check your schedule for exact date. Minerals are the building blocks of rocks and rocks are the building blocks of Earth and other rocky planets. We use the presence and composition of minerals and mineral assemblages to figure out the geological history of a rock. For sedimentary rocks, we can use the minerals to help understand where the sediments that make up the rock came from, e.g., how far did the sediments travel, how fast was the water flowing that carried them, and what were the original rocks that eroded to create the sediments? For igneous rocks, the minerals help us determine the pressures and temperatures that formed the rocks, what the bulk composition of the rock was and whether there was more than one melting and mixing event during the formation of the rock. The minerals in metamorphic rocks can record the changes in pressure and temperature during a rock’s history, which is useful in understanding the path the rock took during its burial, heating and uplifting back to Earth’s surface. 2 Lab manual for GEOG1120 PHYSICAL PROPERTIES OF MINERALS Minerals are chemical compounds and it is possible to identify them by x-ray or chemical analyses. This, however, is neither practical nor necessary in an introductory study of minerals. Most common minerals can be identified by observing certain readily determined physical characteristics or properties such as colour, hardness, lustre, specific gravity, etc. By making a few simple observations and tests on a specimen in conjunction with an identification table or key, a common mineral can be readily identified. Thus it is necessary for the student of elementary mineralogy to become familiar with the various physical properties in order to successfully identify the common minerals you will see in this course. The physical properties of minerals are sometimes divided into two groups: (a) General physical properties: those physical properties which all minerals exhibit, such as colour, hardness, lustre, etc. (b) Special or specific physical properties: those properties found only in a few minerals such as taste, magnetism, odour, etc. A mineral may be identified on the basis of its general physical properties alone but very often one of the special properties may by the diagnostic one which makes identification possible. GENERAL PHYSICAL PROPERTIES The following list gives some of the physical properties commonly employed for identifying minerals in both the laboratory and the field. (a) Colour Colour is one of the most obvious characteristics of a mineral and is determined by examining a fresh surface in reflected light. For some minerals colour is diagnostic (eg, galena [grey], azurite [blue], olivine [green]) whereas others (eg, quartz) may exhibit a wide range of colours, due to either slight differences in chemical composition or minor amounts of impurities in the minerals. Because fresh and weathered surfaces of a mineral may have different colours (especially among minerals having a tendency to tarnish – see below) it is important to note the type of surface being viewed when determining colour. (b) Lustre The quality and intensity of the light that is reflected from a fresh surface of a mineral is its lustre. Frequently we describe the lustre of a mineral by comparing it to some familiar substance. Two main categories of lustre are recognized in minerals: metallic and nonmetallic. There is no sharp division between them and minerals with lustre intermediate between them are referred to as having sub-metallic lustre. 3 Lab manual for GEOG1120 Metallic lustre: minerals that look like a metal are said to have metallic lustre. They are opaque or nearly so (see also diaphaneity). Non-metallic lustre: several varieties are recognized. Vitreous having the lustre of glass or broken glass. This is a very common type of lustre among rock forming minerals. Resinous lustre like that of resin Silky the lustre of silk; often displayed by minerals composed of fibrous aggregates arranged in parallel fashion. Pearly the lustre of pearl; often displayed by minerals with layer structure such as talc and the micas. Dull or earthy some minerals, because of their weathered or porous surface, scatter incident light so completely they lack lustre (c) Hardness The hardness (H) of a mineral is generally defined as its resistance to abrasion or scratching. Hardness is related to the crystal structure of a mineral and the strength of the bond between adjacent atoms. Hardness may be roughly determined by attempting to scratch a mineral of unknown hardness with one of known hardness, or by using the unknown mineral to try to scratch material of known hardness. Mineral hardness is measured on a relative scale called the Mohs Scale of Hardness (after German mineralogist Friedrich Mohs), which consists of ten common minerals arranged in order of their increasing hardness as follows: Hardness 1 2 3 4 5 6 7 8 9 10 Mineral Talc Gypsum Calcite Fluorite Apatite Orthoclase Quartz Topaz Corundum Diamond (hardness of a fingernail is 2 to nearly 3) (hardness of a penny is slightly less than 3) (hardness of a needle, glass plate are 5.5) (hardness of a streak plate is 6.5) When using a glass plate to determine hardness do not hold it in your hand buy lay it flat on the table surface and draw the mineral across the plate. Check to make sure that what appears as a scratch on the glass is not some of the mineral, which has rubbed off on the glass and can be wiped off with the finger. 4 Lab manual for GEOG1120 Generally hardness is a reliable physical property but variations may occur due to variations in composition. Hardness may also vary with crystallographic direction in a mineral. Since weathering affects hardness, all hardness tests should be made on a fresh surface. (d) Streak Streak is the colour of the finely powdered mineral. It is obtained by crushing, filing or scratching the mineral or by rubbing the specimen on a pieced of unglazed porcelain – called a streak plate. (A streak plate cannot be used with all minerals since some minerals are harder than the plate.) Some minerals have a streak that is the same colour as the hand specimen; others have a streak that differs in colour from the hand specimen and may be diagnostic for the identification of that mineral. (e) Tenacity The resistance a mineral offers to breaking, crushing, bending or cutting is referred to as tenacity. Some terms used to describe tenacity are: Brittle Malleable Sectile Flexible Elastic minerals which break into angular fragments when struck with a hammer (eg, quartz) minerals whose shape can be changed without breaking (eg, native copper) minerals that can be cut into thin shavings with a knife, the pieces do not disintegrate to powder minerals, which will bend without breaking, and remain bent after the bending force is removed (eg, talc, thin plates of gypsum) minerals, which will bend easily and spring back to their original form when the force is released (eg, micas) (f) Diaphaneity The ability of a mineral to transmit light is referred to as diaphaneity and is usually expressed as: Transparent Translucent Opaque objects are clearly visible when viewed through the mineral mineral transmits some light (eg, along the edges) but objects cannot be clearly seen through the mineral or they are distorted minerals which allow no light to pass through, even on the thinnest edge 5 Lab manual for GEOG1120 (g) Cleavage Cleavage is the tendency of a mineral to break preferentially along certain planes yielding a smooth lustrous surface. Minerals possess cleavage because the bonds, which hold the atoms together are not of equal strength in all directions. Some minerals have no cleavage (they show fracture) while others may exhibit one, two, three, four, or six cleavage directions. The number of cleavage planes as well as the angle at which they intersect are diagnostic for any mineral which possesses cleavage. Types of cleavage one cleavage plane; usually called basal cleavage; perfect cleavage in one direction as in micas ii) two cleavage planes; usually called prismatic cleavage; in feldspars and pyroxene (augite) cleavage surfaces intersect at about 90°; in amphibole (hornblende) cleavage planes intersect at about 60° and 120° iii) three cleavage planes; two types are recognized in this group: a. cubic cleavage; minerals with three cleavage planes intersecting at 90° b. rhombohedral (or rhombic) cleavage; minerals with three cleavage planes which do not intersect at 90°; minerals break into six-sided prism with each side having the shape of a parallelogram iv) a few minerals have more than three planes of cleavage; four sets of parallel cleavage surfaces in the form of an octahedron produce octahedral cleavage (eg, fluorite) i) Figure 1 Common types of cleavages in minerals. Also, refer to the displays in the lab as you try to distinguish between the different types of cleavage. 6 Lab manual for GEOG1120 The cleavage planes in some minerals are so well developed that they are easily detected. In other cases cleavage planes may occur in step-like fashion and be so discontinuous as to escape detection by casual inspection. However, if such a specimen is slowly rotated in a good light source, in certain positions, parts of the specimen will reflect light in the same way as large smooth cleavage surfaces. (h) Fracture In some minerals bonding between atoms is so uniform that there is no preferred direction of breakage. Minerals without cleavage are said to fracture. Common types of fractures are: Conchoidal Uneven or irregular Fibrous or splintery Even fracture surface is smooth and rounded (like the surface of a shell or conch) and frequently shows fine concentric ridges; eg, natural glass (obsidian) and flint fracture surface is irregular and rough; many minerals exhibit this type of fracture fracture surface is roughened by splinters or fibres fracture surface, though rough with numerous small elevations and depressions, still approximates a plane surface (i) Specific Gravity The specific gravity (G) of a mineral is a number, which represents the ratio of the weight of the mineral to the weight of an equal volume of water. The specific gravity of a mineral can be determined accurately with suitable equipment but for the purposes of identifying minerals in the laboratory specific gravity can be determined with sufficient accuracy simply by lifting the specimen in your hand. Light coloured silicate minerals have an average specific gravity of 2.6 whereas minerals with a metallic lustre range from 4.5 to 7.0 or 8.0. (j) Crystal Form When a mineral is allowed to grow in an unrestricted environment (ie, not crowded by its neighbours) it will form a crystal bounded by smooth crystal faces. The geometric form (cube, octahedron, prism, etc.) of the crystal is a reflection of the internal (atomic) structure of the mineral and can be used to identify many mineral species. It is important to remember that natural crystals may be distorted by the unequal development of different crystal faces and such crystals will not show their diagnostic crystal form. However, in identifying distorted crystals it should be kept in mind that although the appearance is unusual, the interfacial angles remain the same as in perfect crystals. The interfacial angle is obtained by measuring the angle between one crystal face and the other extended. 7 Lab manual for GEOG1120 Figure 2 Common types of crystal forms of minerals. (k) Crystalline Aggregates Most mineral specimens are aggregates of imperfect crystals. This is largely due to conditions of growth in which crowding seriously interferes with the normal development of any single individual. In aggregates of crystals the individual crystals may be elongate (eg, fibrous form of asbestos) or occur as flattened plates (lamellae). Aggregates of small lamellae are often termed micaceous (eg, one variety of hematite). A mineral, which comprises an aggregate of many small equidimensional crystals is described as granular. 8 Lab manual for GEOG1120 SPECIFIC PHYSICAL PROPERTIES (a) Magnetism Minerals, which in their natural state are attracted to a magnet are said to be magnetic. Magnetite is the only common mineral that is readily attracted by a small hand magnet. Pyrrhotite is also magnetic. Some specimens of magnetite act as natural magnets and will attract iron filings when suspended. Such specimens will orient themselves with their long axis pointing N-S. This variety of magnetite is called lodestone and was used in the earliest forms of compasses. (b) Double Refraction If an object appears double when viewed through a transparent mineral that mineral is said to have double refraction. This is due to the fact that the light passing through the crystal or cleavage fragment is broken into two rays travelling with different velocities and thus having different indices of refraction. Calcite is the only common mineral to show double refraction. (c) Feel The feel of a mineral is the impression one gains by handling or rubbing the specimen. Common terms used to describe feel are smooth, greasy, soapy, etc. (d) Taste Certain minerals are sufficiently soluble (eg, halite or common salt) to be identified by taste. (e) Odour Certain minerals give off a characteristic odour when damp. Kaolinite, for example, has an earthy or dank odour when moistened by exhaling on the mineral. (f) Reaction with acid Certain minerals (especially some of the carbonate group) will effervesce (bubble) when treated with dilute hydrochloric acid. 9 Lab manual for GEOG1120 (g) Twinning A few common minerals, especially members of the plagioclase feldspar group, have parallel, thread-lines or narrow bands running across their cleavage surfaces. These lines are twinning striations, which mark the boundary between several intergrown or twinned crystals. (h) Tarnish Some metallic minerals have characteristic tarnish on a weathered surface. For example, chalcopyrite displays a “peacock” tarnish showing purplish-blue colours. 10 Lab manual for GEOG1120 IDENTIFICATION OF MINERALS This laboratory exercise is designed to acquaint you with the common rock forming minerals and minerals of economic importance. It is essential that you become familiar with the rock forming minerals, as you will encounter them again in subsequent labs on igneous, sedimentary and metamorphic rocks. Use the flow charts in Figures 3 to 5 in conjunction with Mineral Identification Tables on pages 12-14 to help you determine the physical properties and name of each of the unknown mineral samples in the lab. The flow charts and the mineral identification tables will assist you in naming the minerals in the study set. The charts are arranged in a systematic manner. First, determine whether the mineral’s lustre is metallic or non-metallic. Then determine the streak, the mineral’s hardness, number of cleavage planes and angles between them, if possible and other properties as needed (magnetism, reaction with acid, etc). Figure 3 Identification key for minerals with metallic lustre. 11 Lab manual for GEOG1120 Figure 4 Identification key for dark-coloured minerals with non-metallic lustre. Figure 5 Identification key for light-coloured minerals with non-metallic lustre. 12 Lab manual for GEOG1120 Physical Properties of Some Common Minerals Table 1 MINERALS WITH METALLIC LUSTRE 1. Hardness less than 2.5 (can be scratched with fingernail) Streak Cleavage/Fracture H S. Other Mineral G. (composition) black perfect cleavage in one 1 2 dark grey black; Graphite direction greasy feel, leaves a C mark on paper grey 3 directions at right 2.5 7.5 lead grey; commonly Galena angles in cubic crystals PbS yellow- * 1-5 3.6 occasionally in Limonite brown - masses with metallic Fe(OH)n(H2O) 4.0 lustre; more commonly as earthy masses 2. Hardness between 2.5 and 5.5 (harder than a fingernail, softer than glass) Streak Cleavage/Fracture H S. Other Mineral G. (composition) black * 3.5 4 brassy yellow; often Chalcopyrite -4 tarnished; similar to CuFeS2 pyrite but not in cubic crystals yellow 6 directions; only a few 3.5 4 most are resinous; Sphalerite are usually present in a only a few varieties ZnS single specimen have submetallic lustre; brown, yellow, black and colourless varieties 3. Hardness greater than 5.5 (harder than a knife) Streak Cleavage/Fracture H S. Other Mineral G. (composition) dark no cleavage 65 brassy yellow; Pyrite grey6.5 commonly in cubes or FeS2 black 12-sided crystals with to striated faces; also in black granular masses black * 6 5.2 iron black; strongly Magnetite magnetic Fe3O4 red* 5.5 5.3 Only the rare black Hematite brown variety has metallic Fe2O3 6.5 lustre, specular or earthy lustre * This property is rarely seen or measurable. 13 Lab manual for GEOG1120 Table 2 MINERALS WITH NON-METALLIC LUSTRE 1. Hardness less than 2.5 (can be scratched with fingernail) Streak Cleavage/Fracture H S. Other Mineral G. (composition) White perfect cleavage in one 2 2.3 vitreous to pearly; Gypsum direction, two other colourless CaSO42H2O good transparent variety is or selenite; fine-grained massive is alabaster; white, yellow, grey, red, brown colourless perfect cleavage in one 22.8 colourless mica; can Muscovite direction 2.5 be peeled into KAl2(AlSi3O10) transparent sheets (OH)2 2. Hardness between 2.5 and 5.5 (harder than a fingernail, softer than glass) Streak Cleavage/Fracture H S.G. Other Mineral (composition) green or perfect cleavage in 2.5 2.7- dark-coloured mica; Biotite brown one direction -3 3.3 usually brown or K(Fe, Mg)3 green; can be peeled (AlSi3O10) (OH, into thin sheets F)2 * 1-5 3.6-4 commonly as earthy, Limonite powdery masses; and FeO(OH)n(H2 as coatings on other O) yellowminerals brown 6 directions; only a 3.5 4 resinous (may have Sphalerite few are usually yellow streak) to ZnS present in a single submetallic lustre; specimen colours include white, colourless, black, brown 3 directions at 2.5 2.2 colourless to white; Halite white right angle cubic crystals; salty NaCl taste 3 directions, not at 3 2.7 varicoloured; usually Calcite or right angle white or colourless; CaCO3 rhombic or elongate crystals; reacts with HCl colourless 4 directions 4 3.2 Cubic crystals; Fluorite varicoloured; light CaF green, purple, blue, yellow; colourless 14 Lab manual for GEOG1120 3. Hardness between 5.5 and 6.5 (harder than knife, softer than streak plate) Streak Cleavage/Fracture H S.G. Other Mineral (composition) red* 5.5 5.3 reddish-brown; often Hematite brown found in earthy masses Fe2O3 6.5 (softer) 2 directions at 6 2.5- colour variable Orthoclase white right angle 2.6 (commonly salmonFeldspar pink); pearly-glassy KAlSi3O8 lustre 2 directions at 6 2.5- Pearly-glassy lustre; Plagioclase or right angle 2.6 striations on some Feldspar cleavage surfaces; Ca(Al2Si2O8) colour varies with Ca NaAlSi3O8 colour and Na amounts less 2 directions at 56° 5-6 3-3.4 amphibole with double Hornblende and 124° chain structure; dark Chain silicate green to black; with Ca, Na, Fe, to prismatic crystals Mg, Al, OH 2 directions at 87° 5-6 3.2- pyroxene with single Augite pale and 93° 3.3 chain structure; dark Chain silicate green green to black; with Ca, Na, Fe, prismatic crystals Mg, Al 4. Hardness greater than 6.5 (harder than a streak plate) Streak Cleavage/Fracture H S.G. Other Mineral (composition) 6.5 3.2- granular masses; Olivine mineral -7 4.4 stubby prismatic (Mg,Fe)2SiO4 crystals; transparent to scratches translucent; yellow to olive green or brown to streak black; vitreous conchoidal plate fracture 7 3.5- equidimensional 12Garnet family 4.3 sided crystals; Complex Ca, Fe, but varicoloured; red, Mg, Al, Cr, Mn green, brown, black silicate doesn’t 7 2.65 Elongate 6-sided Quartz prisms; wide range of SiO2 powder colour; colourless, pink, purple, black, yellow, green * This property is rarely seen or measurable. 15 Lab manual for GEOG1120 ROCKS In the next series of labs you will become familiar with the different types of igneous, sedimentary, and metamorphic rocks. You will become adept at recognizing different types of rocks and be able to classify them according to their texture and mineral composition. Your previous experience with identifying minerals will be essential in order to recognize the most common types of rocks. This section of the lab will be organized as follows: Lab 3 Igneous rocks. You will learn how igneous rocks form and what they tell us of the parent magma and subsequent cooling history. You will become familiar with the most common and/or distinctive types of igneous rocks in hand specimens. Lab 4 Sedimentary rocks and weathering and sediments. Hand specimens of the major types of sedimentary rocks (clastic and chemical) will be provided. You will learn how to recognize the different categories of each based on information on grain size and mineral composition, and to infer from this the environment of deposition. You will also look at sediments and the effects of weathering and erosion. You will try to infer information concerning the source, mode of transport and depositional environment. Lab 5 Metamorphic Rocks. You will be able to see from the samples provided how the processes of metamorphism change the nature, size, and orientation of minerals in a rock. You will learn the dominant textures and minerals of metamorphic rocks, and what these tell us of the amount of heat and/or pressure that the parent rock has undergone. 16 Lab manual for GEOG1120 THE ROCK CYCLE Rocks are naturally occurring aggregates of minerals. Different rock types can be recognized on the basis of their physical properties, texture, and mineral composition. Three fundamental rock categories are recognized: Igneous rocks result from the cooling and crystallization of molten earth materials. Sedimentary rocks result from the erosion, transport, deposition, and lithification of detrital material, or from biological/chemical precipitation. Metamorphic rocks result from the alteration of pre-existing rocks by heat, pressure, and mechanical deformation. With reference to the rock cycle illustrated below, we can see how each type of rock is related to the others. It is important to realize that any rock can be derived from, and give rise to, any other rock. The rock that you hold in your hand is not the final product; given enough time, it will eventually be changed into something else. Figure 6 The rock cycle. 17 Lab manual for GEOG1120 IGNEOUS ROCKS A rock is any natural aggregate of minerals (eg, granite), glass (eg, obsidian), or organic particles (eg, coal). Magma is molten rock beneath Earth’s surface that has been forced to become fluid by the intense heat within the planet. If magma flows onto the surface of the land, or onto the seafloor, then it is termed lava. Igneous rocks are of two types; rocks that form from cooling of magma beneath Earth’s surface, intrusive coarse-grained rocks, and rocks that form from cooling of lava at Earth’s surface, extrusive fine-grained rocks. Dike rocks are intermediate in grain size between intrusive and extrusive rocks, but in this lab we will consider dike rocks as intrusive. Igneous rocks are classified and named by looking at their grain sizes, textures, and the compositions both of the initial melt and the final cooled mineral assemblages. In order to understand igneous rocks and their modes of formation, you should be aware of the following terms and definitions. TEXTURES OF IGNEOUS ROCKS Texture is one of the most important attributes of igneous rocks and refers to the shape, size, and arrangement of the mineral grains or crystals in the rocks. (a) Glassy texture If magma is cooled very rapidly, such as into seawater or glacial ice, there is no time for any ions within the magma to form a crystalline structure. The result is a non-crystalline, glassy volcanic rock called obsidian. Although obsidian is black in colour, it is usually a felsic rock, derived from a granitic magma. (b) Vesicular texture A lava which contains abundant gases may cool while the gases are still bubbling out, producing a rock which is also very porous or vesicular. Scoria and pumice are examples of rocks with abundant vesicles. Silica-rich lavas are very viscous and gases within them have a hard time bubbling out. Instead, the bubbles churn up the lava and when it solidifies it forms pumice, a volcanic rock that is so porous, it floats. Like obsidian, pumice is formed from glass particles. 18 Lab manual for GEOG1120 The size of the vesicles can range from 1 mm to several centimetres in diameter. Vesicles that are filled with secondary mineral matter precipitated by percolating water are called amygdules and the texture is amygdaloidal. (c) Fragmental texture A volcanic explosion results in materials (molten lava, ash and broken rock debris) being thrown into the air and then settling back onto land or water. Rocks formed in this manner have a pyroclastic texture. The fragments in pyroclastic rocks have irregular shapes and sizes. The angular fragments are called pyroclasts, and are classified in order of grain size: i) ash; particles < 2 mm; when heat melts the ash particles together, the resulting rock is called a tuff. ii) lapilli; particles 2-64 mm, pebble size iii) volcanic bombs; particles > 64 mm; when lapilli and/or volcanic bombs are lithified the resulting rock is a volcanic breccia. (d) Crystalline texture Crystalline textures are classified on the basis of the sixe of the particles that make up the igneous rocks. Grain size is a direct result of the rate at which magma cools. For example, if magma cools slowly, as in intrusive rocks, mineral crystals have time to grow to a visible size and are called medium (1-2 mm) or coarse grained (> 2 mm). Rocks with mineral crystals from 1 to 10 mm in diameter are called phaneritic (from the Greek word for visible). Granite is an example of a phaneritic rock. If the grains are all the same size, the rock is said to be equigranular. If, on the other hand, magma is extruded onto the surface of Earth, where heat is dissipated away from the crystallizing lava much more quickly, minerals do not have time to grow very large, and the result is a more fine grained (< 1 mm) texture. These volcanic rocks are called aphanitic. Basalt is a rock with aphanitic texture. Some igneous rocks have two distinct sizes of crystals, reflecting a two stage cooling history: one in which magma cooled slowly enough for some large crystals (phenocrysts) to form, and a second stage when the rest of the magma solidified quickly (groundmass). This texture is called porphyritic. Often the very last of the magma to crystallize in a magma chamber is very fluid due to the concentration of volatiles in the residual magma. In this water-rich magma, minerals may grow to very large sizes, and the resulting texture is said to be pegmatitic. Some pegmatites contain minerals over a metre in diameter. They generally occur as veins or dikes. 19 Lab manual for GEOG1120 COMPOSITION AND COLOUR OF IGNEOUS ROCKS The single most important component for the classification of igneous rocks according to composition is silica (SiO2) content. There are four main categories based on the percentage of silica: (a) Felsic rocks >65% SiO2. Felsic minerals are light coloured and rich in silicon and aluminium. They include quartz, potassium feldspar (orthoclase), sodium-calcium feldspar (plagioclase) and muscovite. The magmas that produce these rocks are high in potassium, silicon, and sodium, and low in iron, magnesium, and calcium. Granites and rhyolites are therefore characteristically light coloured. The finer grained volcanic rocks (where individual grains cannot be seen) show a wide variation from white to pink to buff. (b) Intermediate rocks 53-65% SiO2. Quartz is not an essential mineral and is usually <10%. There is more plagioclase than orthoclase. The ferromagnesian minerals are biotite and hornblende and minor pyroxene may be present. Diorites and andesites are characteristically intermediate in colour. Andesites can be shades of rust, grey, green, or purple. (c) Mafic rocks 45-53% SiO2. The predominant minerals are pyroxene and calcium-rich plagioclase (dark grey-blue colour). The magmas that produce these rocks (gabbros and basalts) are relatively high in iron, magnesium, and calcium, but deficient in silica. Rock colouration is characteristically black or dark green. (d) Ultramafic rocks <45% SiO2. The predominant minerals are pyroxene and olivine. Calcium-rich plagioclase may be present in minor amounts but is not light coloured as it is in felsic rocks. It has a dark grey-blue colour. The mineral constituents of an igneous rock impart a characteristic colour to it. Hence, rock colour is used as a first-order approximation in establishing the general mineralogical composition of an igneous rock. The variations in silica-rich minerals and ferromagnesian minerals result in differing colours of rocks. Figure 7 can be used as an aid to visually estimate the percentage of dark minerals present (which is inversely proportional to the silica content). Felsic: <30% dark minerals Intermediate: 30-60% dark minerals Mafic: 50-60% dark minerals Ultramafic: >60% dark minerals 20 Lab manual for GEOG1120 Figure 7 Comparison chart for visual percentage estimation (after Terry and Chilingar, 1955) As you try to determine the mineralogical composition of the rocks, keep in mind that you are trying to assess three major mineralogical criteria: 1. Presence or absence of quartz. Quartz is an essential mineral in felsic rocks and an accessory mineral in intermediate or mafic rocks. 2. Composition of the feldspars. K-feldspar and Na-plagioclase are essential minerals in felsic rocks but are rare or absent in intermediate and mafic rocks. Calcium plagioclase is characteristic of mafic rocks. 3. Proportion and kinds of ferromagnesian minerals. Mafic rocks are rich in ferromagnesian minerals, while felsic rocks are depleted in these minerals. Olivine is generally restricted to mafic rocks. Pyroxenes and amphiboles are present in mafic to intermediate rocks. Biotite is common in intermediate and felsic rocks. 21 Lab manual for GEOG1120 Hints on Mineral Identification in Hand Specimens Identification of the major rock types may be difficult at first for the beginning student. The following is a brief summary of some minerals and how they may appear in igneous rocks. Use of a hand lens and a microscope generally are a great help. Relatively few minerals make up most igneous rocks. To correctly identify the rock, you must identify the major minerals. This is fairly easy for coarse-grained rocks, more difficult (but not necessarily impossible) for fine-grained rocks, and impossible for glassy rocks (unless phenocrysts are present). Quartz K-feldspar Plagioclase Muscovite Biotite Amphibole Pyroxene Olivine Occurs as irregular, glassy grains commonly clear to smoky in colour. No cleavage, but conchoidal fracture may be seen on some surfaces. A hand lens is useful in distinguishing quartz from light coloured feldspars, which appear milky or translucent. Porcelain lustre. Commonly coloured pink, white, or grey. Cleavage in two directions at right angles may be detected. Cleavage planes flash light when specimen is rotated. Usually grey or white in granite, dark bluish colour in gabbro. Striations common. Two cleavage directions at right angles may be detected. Brass-coloured flakes associated with quartz or K-feldspar. Perfect cleavage in one direction. Sometimes glitter like small specks of pyrite. Small black flakes. Perfect cleavage in one direction. Reflects light. Can be distinguished from hornblende or pyroxene by scratching or crushing with a steel probe. Long, black crystals in a light coloured matrix. Cleavage at 60° and 120°. Short, dull, greenish black minerals in darker rocks. Cleavage in two directions at right angles. If grain size is very fine, it may be difficult to distinguish between pyroxene and amphibole. If this is the case, it is appropriate to put “presence of pyroxene and/or amphibole” in your description. Glassy, light green (sometimes yellow or light orange) stubby crystals massed together. Conchoidal fracture on some grains. NOTES: Most dark-coloured igneous rocks are rich in calcium plagioclase and ferromagnesian (ironmagnesium) minerals such as pyroxene and olivine. The word mafic refers to such rocks. Ultramafic igneous rocks are composed entirely of ferromagnesian minerals. Light-coloured or felsic igneous rocks commonly contain potassium feldspar, sodium plagioclase, and quartz. Intermediate igneous rocks are neither dark nor light and generally contain light-coloured minerals (feldspars, some quartz) and dark minerals such as hornblende or biotite. Pink feldspar is usually potassium feldspar, white or grey feldspars may be either potassium feldspar or plagioclase – if twinning striations are present, it is plagioclase. Amphibole cleavages do not intersect at 90°, but pyroxene cleavages do, amphibole typically are elongate and have a splintery appearance whereas pyroxenes look blocky. 22 Lab manual for GEOG1120 Figure 8 The names of common igneous rocks are based on the minerals and texture of a rock. A mineral’s abundance in a rock is proportional to the thickness of its band beneath the rock name. Magma Composition and Tectonic Setting The composition of igneous rocks can vary between two end members: a light coloured, acid (silica-rich) syenite and granite/rhyolite a dark coloured, basic (silica-poor) gabbro/basalt These end members represent extremes in the compositional variation of the parent magma, which in turn reflects tectonic setting in which melting took place. Granitic magmas tend to form in collisional plate boundaries (subduction zones) where oceanic crust and overlying sediments descend into the mantle, gradually heating up until the melting point of silica-rich minerals is reached (around 600° C). This silica-rich magma then rises to produce deep-seated granitic intrusions and rhyolitic lava flows at the surface. Similarly, at the root of mountain belts where high pressures are found, the temperature can rise up to the melting point of silica-rich minerals and melt some of the deformed (metamorphosed) rocks to produce granitic magmas. Andesitic magmas also form at collisional plate boundaries. The magmas at subduction zones form partly from a mixture of seafloor sediments and partly from melting of basaltic and felsic crust. 23 Lab manual for GEOG1120 Basaltic magmas are formed at divergent plate boundaries as a result of the melting of the asthenosphere. The asthenosphere is made up of peridotite, an ultrabasic, ultramafic, ferromagnesian-rich rock that melts at a much higher temperature (around 1000° C) and subsequently rises to the surface and solidifies as basaltic oceanic crust. PROCEDURE FOR IGNEOUS ROCK IDENTIFICATION Examine the numbered samples and fill out the Igneous Rock Identification forms in your lab handout using the method below, the hints on page 22 and Figure 8 on page 23. 1. Examine the rock and determine the colour. You can refer to the rock as: a. felsic - few dark minerals, generally a light colour b. intermediate - nearly 50% dark minerals c. mafic - over 70% dark minerals, very dark colour 2. Examine the rock and determine the type of texture. You can then refer to the rock as: a. glassy – smooth or frothy b. pyroclastic – ash, lapilli or breccia c. crystalline – aphanitic, phaneritic or porphyritic; specify the type of minerals that make up the phenocrysts, if present d. vesicular e. amygdaloidal f. pegmatitic 3. Determine the approximate percentage of quartz: a. 10-40% quartz; granite-rhyolite family b. <10% quartz; diorite-andesite family c. no quartz; gabbro-basalt family 4. Determine the approximate percentage and type of feldspar: a. pink feldspar is almost invariably a potassium (K) feldspar b. white or grey feldspar may be either potassium feldspar or plagioclase c. if they feldspar has striations it is definitely plagioclase. 5. Determine the percentage and type of dark minerals in the rock. 6. Use Figure 8 on page 23 and determine the rock name. Referring to the notes in this lab, place each of your numbered unknown samples in a tectonic setting. Intrusive bodies are coarse-grained because they were cooled deep within the crust. How can intrusive rocks now be exposed at Earth’s surface? 24 Lab manual for GEOG1120 SEDIMENTARY ROCKS AND WEATHERING AND SEDIMENTS INTRODUCTION Sedimentary rocks are produced on the surface of Earth by a chain of processes that includes weathering, erosion, transportation, deposition, and diagenesis. Sedimentary rocks are important for several reasons: 1. Although the volume of sedimentary rocks in Earth’s crust is small relative to igneous and metamorphic rocks, they cover over 2/3 of Earth’s surface. 2. Sedimentary rocks contain virtually the entire record of Earth’s past life in the form of plant and animal fossils. Igneous and metamorphic rocks, on the other hand, form at temperatures too high and in environments not suited for living organisms or for the preservation of fossil remains. 3. Sedimentary rocks are economically important because they contain almost all of our fossil fuels (petroleum, natural gas, and coal), and many important ore deposits (uranium, copper, lead, zinc, iron, manganese). The purpose of this lab is to provide you with some insight into the processes that produce sedimentary rocks and to help you identify the common sedimentary rocks. Your lab assignment will consist of examining unknown rocks and recording their textural and compositional characteristics. There are three fundamental types of sedimentary rocks, based on textural features and particle size: Inorganic detrital Chemical precipitates Biogenic (organic) derived from particles (clasts) of minerals and rocks that have been separated from the parent material largely by processes of physical weathering. derived from the dissolved products of chemical weathering, which have precipitated from solution by inorganic processes (evaporates, travertine). particles are derived from life activities of organisms (eg, shell fragments, coal, chalk, chert). We will look at the textures and compositions of rocks within each of these groups. 25 Lab manual for GEOG1120 FORMATION OF SEDIMENTARY ROCKS The four steps required to make sedimentary rocks: 1. 2. 3. 4. Weathering produces loose particles (sediment) from previously existing rocks. Transportation moves those particles to an area where they can be collected. Deposition causes the particles to settle to the bottom of the collecting basin. Lithification transforms the sediment into a cohesive, solid rock again, via various processes that may include compaction, cementation, and recrystallization. Figure 9 The processes involved to form sediments and sedimentary rocks. 26 Lab manual for GEOG1120 INORGANIC DETRITAL (CLASTIC) ROCKS Textures of Clastic Rocks Texture in sedimentary rocks is important because it provides important clues concerning the distance that the sediment has been transported and the environment in which it was deposited. The factors that define the texture of clastic rocks are grain size, rounding, sorting, compaction, and cementation. (a) Grain Size The size of particles in clastic rocks ranges from large blocks several metres in diameter to fine dust. A simple classification of clastic rocks is based on grain size: Gravel Sand Silt or Clay > 2 mm in diameter. Rocks, which contain clasts in this size category are termed conglomerates (rounded clasts) or breccias (angular clasts). 1/16 to 2 mm diameter. Rocks, which contain clasts in this size category are termed sandstones. < 1/16 mm in diameter. Rocks with grains in this size category are called mudstones or if fissile (split easily) shales. Includes grains from 1/256 to 1/16 mm (silt) and grains less than 1/256 mm (clay). (b) Rounding Rounding refers to the smoothness of the grains, which is a result of the amount of abrasion that the grains have undergone during transport. Sediment moved by ice or by the direct action of gravity is commonly angular, whereas particles carried by wind or water are rounded by continual abrasion. Also, large, angular boulders indicate a nearby source area in a mountainous terrain, as significant transport by streams rapidly rounds off the rock corners and wears down the boulder. Figure 10 Terms used to define roundness of sediment. It is best to observe this characteristic with a hand lens or microscope. 27 Lab manual for GEOG1120 (c) Sorting Sorting is a measure of the range of grain sizes present in the rock. It is a very important textural characteristic because it can provide clues concerning the history of transportation and the environment in which the sediment accumulated. Well sorted material comprises grains of one dominant size, and usually of one type of composition. This usually reflects a uniformity of current flow, usually by wind or streams. Poorly sorted material contains grains of several different grain sizes. Glaciers and mudflows typically deposit fine and coarse materials together, hence they are poor sorting agents. Figure 11 Terms used to describe sorting in sediments. (d) Compaction Compaction occurs as sediments are built up and the pressure of the overlying beds squeezes water and air out of the loose sediment, often causing partial dissolution and reprecipitation of minerals on the contacts between the grains. This helps to bind the sediments together, one way by which unconsolidated sediment is converted to rock. (e) Cementation Cementation is the process by which mineral grains or shell fragments are bound together by minerals precipitated from groundwater in pore spaces between the clasts. The most common natural cements are calcite, quartz, hematite, and limonite. A rock’s reddish or brown colour is usually enough to identify the iron oxide minerals (hematite and limonite) as cements. Calcite cement is easily identified using a drop of acid. Silica cement results in a very hard rock from which individual grains are dislodged only with some difficulty. 28 Lab manual for GEOG1120 Composition of Clastic Rocks The majority of clasts in clastic sedimentary rocks are composed of a few common constituents, most of which you have seen in previous labs. These are: Minerals quartz, feldspars, less common silicates (eg, micas, garnet), clay minerals (eg, kaolinite), iron oxides (magnetite, hematite) Lithic clasts fragments of other rocks Biogenic clasts organically produced fragments such as shells or plant remains Maturity of Clastic Rocks Maturity is a term that is used to describe the stability of the sediments in a clastic rock, which may be related in part to the distance the sediment has been transported or the time that has elapsed between weathering and deposition. The rounding, sorting, and mineral composition of clastic rocks all can be used to describe the maturity of a sediment. Chemically mature rocks are those, which contain large quantities of quartz or clay. Quartz is the most stable of all silicate minerals, whereas the ferromagnesians (like olivine, pyroxene, biotite) having higher crystallization temperatures, alter rapidly to clay minerals under surface weathering conditions. The dominant presence of quartz or clay in a sediment, therefore, suggests that considerable time has passed in order for the other less stable silicate minerals to break down. Mechanical maturity is exhibited by sediments which have undergone considerable abrasion during transportation, or have been deposited a long distance from their source. Thus a well-rounded, well-sorted sandstone is considered mechanically mature, whereas a poorly-sorted, angular breccia is considered mechanically immature. Mudstone and quartz sandstone are the most mature of all clastic rocks, being well sorted and composed largely of stable minerals (clay and quartz). The concept of maturity is not easily applied to, nor useful for, classifying chemical sedimentary rocks. 29 Lab manual for GEOG1120 Grain size class and diameter gravel (> 2 mm) sand (1/16 – 2 mm) Composition quartz, feldspar, rock fragments, and clay minerals mostly quartz feldspar >25% mostly lithic fragments >50% silt (1/2561/16 mm) and <50% clay (<1/256 mm) >50% clay (<1/256 mm) and <50% silt (1/256-1/16 mm) fine quartz grains, clays Texture rounded grains angular grains all types of rounding and sorting usually poorly to moderately sorted usually poorly sorted nonfissile (compact) fissile (splits easily) Rock name CONGLOMERATE BRECCIA QUARTZ SANDSTONE ARKOSIC SANDSTONE LITHIC SANDSTONE SILTSTONE SHALEY SILTSTONE MUDSTONE clay, organic nonfissile particles (may be coloured by impurities) such as carbon fissile >90% clay (<1/256 CLAYSTONE or (black), reduced mm) SHALE iron (green), or oxidized iron (red) Table 3 Common clastic rocks according to composition and texture. 30 Lab manual for GEOG1120 CHEMICAL ROCKS Textures of Chemical Precipitates (a) Crystalline texture Crystalline texture is in marked contrast to the clastic textures already described. The minerals precipitated from seawater, groundwater, or lakes from this texture, which consists of a network of interlocking crystals similar to the texture in some igneous rocks. However, we can assign a similar size classification to chemical rocks as we do to clastic rocks, substituting the word crystalline for grained. Thus you have the following: coarse crystalline medium crystalline fine or microcrystalline cryptocrystalline > 2mm 1/16 – 2 mm 1/256 – 1/16 mm < 1/256 mm Deposits from springs and cave-formed rocks are commonly microcrystalline with a banded appearance that results from chemical variations and impurities during deposition. (b) Skeletal texture (fossiliferous) Skeletal texture is formed by the accumulation of skeletal parts of invertebrate marine life. Calcium carbonate is removed from seawater by marine organisms to make their shells and other hard parts. When the organisms die, their shell material settles to the sea floor and may be concentrated as shell fragments on a beach or near a reef. The texture of the resulting rock is similar to a classic texture, but the material is unique in that it consists of the skeletal fragments of organisms that originate in the environment of deposition. Skeletal texture predominates in many limestones. (c) Oolitic texture Oolitic texture resembles sand, but the individual grains have concentric layers. This layering is produced when calcium carbonate, precipitated on the sea floor, is agitated by wave or current action and accumulates around a tiny shell fragment or grain of silt. As the particle moves to and fro, thin spherical layers are built up by accretion. Close examination of an oolitic texture reveals a concentric structure around each nucleus, and generally a minor amount of associated shell debris. 31 Lab manual for GEOG1120 Composition of Chemical Rocks Chemical sedimentary rocks are divided into four rock groups, based on their composition and mode of formation. (a) Carbonates Carbonates consist of 50% or more carbonate minerals (calcite and dolomite), and comprise the most abundant group of chemical sedimentary rocks. Their formation is closely linked to the growth and death of organisms. Many aquatic organisms are able to extract calcium carbonate dissolved in the water to form a protective shell or skeleton. When these organisms die their shells or skeletons accumulate as sediments, which later become cemented by calcite. Carbonate rocks, therefore, are often rich in fossils and the fossils may provide information on the depositional environment of the host rock (eg, presence of reef fossils in a limestone suggests that the rock was formed in a shallow marine environment). A sedimentary rock containing abundant visible fossils may have the adjective “fossiliferous” attached to it, as in a fossiliferous limestone. Examples of carbonate rocks are limestones and dolostones. Limestone that has been precipitated from fresh water, such as in springs or geysers, is called travertine. (b) Evaporites Evaporites are composed of minerals, which precipitated out of a solution (eg, seawater) because of solar evaporation. The most common examples of evaporate minerals include halite (NaCl), sylvite (KCl), gypsum (CaSO42H2O), and anhydrite (CaSO4). Even the carbonate minerals calcite and dolomite can form by evaporation, although biochemically produced carbonate is much more abundant. The presence of evaporates usually indicates an environment in which evaporation exceeds precipitation, or in which seawater is prevented from mixing with the open ocean such as might occur in a shallow or restricted sea. (c) Siliceous rocks Siliceous rocks are composed mainly of polymorphs of microcrystalline quartz (SiO2), formed when silica dissolved in seawater precipitates. These include chert (light colour), flint (dark), jasper (red), and agate (banded). Chert commonly occurs as nodules in limestone. Like other varieties of quartz, siliceous rocks are characterized by a hardness of 7 and conchoidal fracture. 32 Lab manual for GEOG1120 Siliceous sedimentary rocks may have either an inorganic or biochemical origin. Migrating groundwater may deposit large amounts of chert in the pore spaces of limestone. Some cherts are thought to be derived from layers of silica in sediments underlying high diatom or radiolarian abundance in the ocean (these are microscopic organisms that extract silica from seawater to build their protective skeletons). (d) Iron formations Iron formations are a group of rocks that formed in the Precambrian Era when Earth’s atmosphere was much lower in oxygen that it is today. They are chemical precipitates of hematite, magnetite, and limonite, and may have a layered, oolitic, or compact structure. Rock group Composition Texture & diagnostic features medium to coarse crystalline = crystalline; white, grey microcrystalline = micrite (lime mud); commonly black Calcite (CaCO3) aggregate of oolites = oolitic Carbonate abundant fossils in calcite matrix = fossiliferous banded calcite = travertine; effervesces with HCl; may be colour banded Dolomite (CaMg[CO3]2) textural varieties similar to limestone; powder effervesces weakly with cold dilute HCl Siliceous Silica (SiO2) cryptocrystalline, dense, with conchoidal fracture Gypsum (CaSO42H2O) fine to coarse crystalline; can be scratched with fingernail; Evaporite white, buff Halite (NaCl) cubic cleavage; massive, granular, compact, colourless, white Iron formation Hematite, magnetite, variable; may be crystalline, limonite oolitic, earthy, or banded Table 4 Chemical sedimentary rocks by composition and texture. Rock name Limestone Dolostone Chert Gypsum Rock Salt Ironstone 33 Lab manual for GEOG1120 BIOGENIC (ORGANIC) ROCKS Organic rocks are not rocks in the strict sense because they are composed primarily of organic material and not minerals. These deposits form in special environments in which either the rate of accumulation of organic debris is very high, or the environment is such that the decomposition of organic matter is very slow. Such environments may include swamps, marshes, or stagnant lagoons. Peat and coal are derived from plant debris that accumulated at various times during Earth’s history. With each successive stage in coal formation, higher temperatures and pressures drive off impurities and volatiles, and result in increasing carbon content, hardness, and lustre. peat (partially altered plant material) Rock group Carbonate Organic burial lignite (soft, brown coal) Composition Calcite (CaCO3) mostly carbon (C) further burial bituminous (soft, black coal) metamorphism Texture & diagnostic features bioclastic; calcareous shell fragments in a massive or crystalline matrix; effervesces with HCl bioclastic; fossils and fossil fragments loosely cemented = Coquina earthy, (bioclastic); shells of microscopic organisms, clay-sized, soft; white; effervesces with HCl fibrous, (bioclastic); earthy plant remains; brown; soft, porous dense, black, varying degrees of lustre and hardness anthracite (hard, black coal) Rock name Limestone Chalk Peat Coal (lignite, bituminous, anthracite) Table 5 Biogenic (organic) sedimentary rocks by composition and texture. SEDIMENTARY STRUCTURES Features called sedimentary structures that formed during transportation or deposition often reveal the environments in which clastic rocks were deposited. 34 Lab manual for GEOG1120 (a) Bedding or Stratification The most common sedimentary structure is a bed, or stratum, a horizontal layer of sediment that represents a deposition under similar conditions over a period of time. A bed differs from those above and below it in ways that may be subtle (slight change in grain shape, size, or sorting) or obvious (eg, change from green to red or black colour, or from mudstone to conglomerate). Many beds are homogenous from top to bottom, but in some cases there is a systematic change in grain size, from coarse at the bottom to fine-grained at the top. This is called a graded bed, and results when a mixture of different sediments is deposited all at one time, such as in a turbidity current. The coarsest material, being heaviest, is deposited first, followed, by increasingly smaller grains, and finally the very fine suspended material. A graded bed is useful to geologists studying highly deformed rocks, because it can be used to tell which side of the bed was originally the top and which was the bottom. Figure 12 A typical graded bed. (b) Cross-bedding (cross-stratification) Most strata are deposited in nearly horizontal sheets. However, some stratification is not horizontal at the time of deposition, and is referred to as cross-bedding. For example, sediment transported in a single direction by water or air currents commonly forms bedding planes that represent the slip faces of dunes or ripples. The following diagrams illustrate the difference in appearance between horizontal and cross-bedding. Figure 13 Horizontal stratification. Schematic vertical profile of a deposit of horizontal strata. Figure 14 Cross-bedding. Nonhorizontal layering that is found in dunes and sand ripples. 35 Lab manual for GEOG1120 (c) Ripple Marks When sand grains are deposited in the presence of either air or water currents, they frequently develop a surface with undulating features, called ripple marks. Ripple marks may be preserved either on the upper surface of the stratum, which exhibits the cross-bedding, or on the lower surface of the overlying bed (which itself may be either horizontally- or cross-stratified). Asymmetric ripple marks are especially useful for determining paleocurrent directions, as the direction of the current is at right angles to the crest of the ripple mark. Figure 15 Asymmetrical ripple marks form when the current flow is always in the same direction, and thus have a steep downstream side and a gently sloped upstream side. Figure 16 Symmetrical ripple marks form where currents move back and forth, as on a tidal flat or nearshore beach face. Both slipfaces on symmetrical ripple marks are equally steep. (d) Sole Marks Sole marks are a general term for sedimentary structures commonly found on the bases of beds formed by scouring or dragging of material in the lower part of the depositional flow. Tool marks are a class of sole marks, which may form on bedding planes when shells, sticks, pebbles, or bones slide, roll or bounce along a surface. Tool marks appear as holes or linear drag marks. Flutes are scoop-shaped or V-shaped depressions scoured into a surface by the erosional, winnowing action of currents. Overlying beds will form natural casts of these depressions; such structures are called flute casts. As is the case with ripple marks, flutes and flute casts may be used to indicate paleocurrent direction. (e) Mud Cracks and Raindrop Impressions Many sedimentary rocks also contain structures that formed shortly after deposition of the sediments that compose them. For example, desiccation cracks often form while moist deposits of mud dry and shrink. These structures are often preserved when an overlying bed of a different lithology penetrates the cracks, creating a network of mudcrack polygons that are distinct in colour or grain size from the surrounding material. Raindrop impressions may form on terrestrial surfaces, usually on a soft material such as mud. The result is numerous small depressions on the upper surface of the bed. 36 Lab manual for GEOG1120 (f) Trace Fossils Animals make tracks, trails, and burrows, which can also be preserved in sedimentary rocks. Such traces of former life are called trace fossils. Figure 17 The three types of trace fossils of trilobites. Trace fossils can provide a lot of information on animal behaviour and allows a better understanding and reconstruction of past life forms. SEDIMENTARY ENVIRONMENTS Sediments are deposited in many different environments. They have been studied in modern situations so that characteristic sediments, sedimentary structures, and fossils are known for each one. The information gained from grain characteristics, sedimentary structures, and fossils can be used to infer what ancient environments were like in comparison to modern ones. The term sedimentary environment refers to the place where the sediment is deposited, and to the physical, chemical, and biological conditions that exist in that place. The following brief summary describes some of the important characteristics of the sedimentary rocks formed in each environment. (a) Alluvial Fans They are stream and sediment gravity flow deposits that accumulate near a mountain front in a dry basin. They typically contain poorly sorted coarse arkosic sandstones, gravel and boulders. These sediments may form in coarsely cross-bedded, lens-like channel deposits. Fine grained sand and silt may be deposited near the margins of the fan. 37 Lab manual for GEOG1120 (b) River Channel and Floodplains Rivers deposit elongate lenses of conglomerate or sandstone in channels. Floodplains are nearly flat expanses across which the rivers of the world typically meander before reaching the sea. A considerable amount of sediment is deposited on these plains. Rocks formed in a floodplain environment are commonly channels of cross-bedded sandstone deposited on the point bar of a meander and enclosed in shale beds, which represent the finer horizontally deposited flood deposits. (c) Lakes They are characterized by thin-bedded shales, perhaps containing fish fossils. If the lake dries up periodically the shales may show mud cracks and evaporite interbeds. (d) Aeolian Environments Wind transports and deposits material in these environments. Wind is an effective sorting agent and will selectively transport sand and dust, leaving gravel behind. Dustsized particles are lifted high in the atmosphere and may be transported thousands of miles to where they accumulate as a blanket of loess. Windblown sand commonly accumulates in dunes that are characterized by well-sorted, fine grains. The dominant sedimentary structure in aeolian environments is large-scale cross-bedding. (e) Glaciers They transport but do not effectively sort material. Thus glacial environments are characterized by unsorted, unstratified accumulations of angular boulders, gravels, sand, and fine silt, called glacial till. There are also fluvial and lacustrine deposits associated with glaciations. (f) Deltas These are large accumulations of sediment that are deposited at the mouths of rivers. A delta is one of the most significant sedimentary environments and includes a number of subenvironments such as stream channels, floodplains, beaches, bars, and tidal flats. The deltaic deposit as a whole consists of a thick accumulation of sand, silt and mud, which fines outwards and downwards (contrast this fining-downward appearance with graded bedding, found in deep ocean environments). (g) Shoreline environments They are characterized by beaches, bars, and spits that commonly develop along low coasts. They may contain quiet water lagoons and tidal flats. The sediment in these features is well washed by wave action and is typically clean, well-sorted quartz sand. Behind the bars and adjacent to the beaches, fine silt and mud are often deposited in tidal flats. 38 Lab manual for GEOG1120 (h) Organic Reef Reefs are solid structures comprising shells and secretions of marine organisms. The reef framework is typically built by corals and algae, but many other types of organisms contribute to the reef community. Together, these organisms produce a highly fossiliferous limestone. (i) Shallow Marine Environments These are found along the continental margins, and in the past were even more extensive than today. Sediments deposited in shallow marine waters form extensive layers of wellsorted sand, shale, and limestone, which typically occur in a cyclical sequence as a result of shifting environments from changes in sea-level. (j) Deep Ocean Environments If they are adjacent to the continents, they receive a considerable amount of sediment transported from the continental margins by turbidity currents. Deep sea deposits are thus characterized by a sequence of graded beds. Figure 18 The main sedimentary depositional environments. 39 Lab manual for GEOG1120 METAMORPHIC ROCKS INTRODUCTION During this lab period you will study a third major group of rocks, the metamorphic rocks. You will learn the minerals and mineral assemblages found in metamorphic rocks and how different metamorphic textures form, and you will see how earth scientists use metamorphic rocks to decipher aspects of Earth’s history. Metamorphic rocks are important because: They are common in regions that have undergone significant deformation. Geologists are interested in these regions because they may mark the positions of plate collisions that took place millions of years ago. As with intrusive igneous rocks, erosion strips off as much as thousands of metres of overlying strata to reveal these rocks that formed deep in Earth’s crust. Vast areas of metamorphic rock form the cores of every continent and are present in linear mountain belts, such as the Appalachians, Rockies, Himalayas, and Alps, which formed as a result of the collision of lithospheric plates. Thus metamorphic rocks reveal the processes by which continents and mountains form. They also record the conditions and processes that occur within Earth, and thus provide clues to the nature of parts of the crust that we have never seen. A number of economically valuable products are either directly or indirectly related to metamorphic processes. In some cases, these products are the rocks themselves, such as marble, slate, or talk; in other cases valuable minerals such as tin and tungsten ores can be extracted from metamorphic rocks. Diamond, the hardest mineral in the world, is the product of intense metamorphism of carbon deep in Earth’s crust. CHARACTERISTICS OF METAMORPHIC ROCKS The process of metamorphism is similar to the processes that form sedimentary rocks in that they observed changes in mineral composition and texture are the result of the rock’s response to different conditions of temperature and pressure. While the processes of chemical weathering proceed under conditions of reduced (surface) temperature and pressure, metamorphic changes occur because of increased temperatures and/or pressures. These conditions are most often found at depth in Earth’s crust. The original rock may be sedimentary, igneous, or an older metamorphic rock (although igneous and metamorphic rocks are generally less affected than sedimentary rocks). In the process, original textural features such as bedding may be obliterated, replaced by a texture, which reflects that metamorphic processes involved. The effects of metamorphism include: 40 Lab manual for GEOG1120 (a) Chemical recombination and growth of new minerals, with or without the addition of new elements from circulating fluids and gases. (b) Deformation and rotation of the constituent mineral grains. (c) Recrystallization of minerals to form larger grains. The net result of any or all of these effects are rocks of greater crystallinity, increased hardness, and new structural features that commonly exhibit the effects of flow or other expressions of deformation. SCALES OF METAMORPHISM There are two main scales at which metamorphic processes occur: (a) Contact Metamorphism Occurs locally beneath lava flows, adjacent to igneous intrusions or along fracture that are in contact with hot gases. Although extremely high temperatures may occur, metamorphism always occurs below the melting point of rocks. If melting occurs, the process in considered igneous. The intensity of contact metamorphism is greatest at the contact between host rock and magma, and decreases rapidly with distance from the magma. Thus zones of contact metamorphism (known as aureoles) are relatively narrow. This aureole or halo of metamorphism is characterized by the formation of hightemperature minerals close to the contact zone and progressively lower temperature minerals as the distance from the contact zone increases. The size of the aureole depends on the temperature and size of the pluton, mineralogy of the host rock, and the presence or absence of fluids. (b) Regional Metamorphism Occurs over extremely large areas and is associated with mountain building. As rocks are buried deep within the crust, they are subjected to changes in temperature, pressure, and chemical conditions. They reach equilibrium with the new conditions by developing in the solid state a new set of minerals that often shows a degree of preferential orientation or foliation. METAMORPHIC GRADE The grade of metamorphism reflects the extent to which a metamorphosed rock is different from the parent rock from which it is derived. The higher the temperatures and pressures to which the parent rock is exposed, the higher the grade of the metamorphosed rock. Mineral assemblages and textures reflect the metamorphic grade 41 Lab manual for GEOG1120 reached while the chemical composition remains essentially unchanged with the exception of water and gasses. The grain size of metamorphic rocks commonly, but not always, increases with grade. The temperature range in which metamorphism occurs is between 100° and 800° C. The range of pressures acting on rocks is more or less unlimited, as pressures increase linearly with depth. However, for the purposes of classification, we use a range of 1-10 kilobars (a bar is equal to standard atmospheric pressure). Thus, one kilobar is equal to one thousand times normal atmospheric pressure. Combining these two parameters, we assign a metamorphic grade to a regionally metamorphosed rock, which tells us the intensity of the temperatures and pressures that have been applied to the rock. Metamorphic Grade Low Intermediate High Temperature 100-300° C 300-500° C 500-800° C Pressure 1-3 kilobars 3-6 kilobars 6-10 kilobars Table 6 Metamorphic pressure and temperature conditions as related to grade. In some cases, temperature and pressure do not increase concurrently. For example, contact metamorphism often occurs under conditions of high temperature and low pressures, such as beneath a lava flow. And at subduction zones metamorphism results from high pressures and (relatively) low temperatures. Figure 19 Some of the environments where metamorphic rocks form. 42 Lab manual for GEOG1120 TEXTURES OF METAMORPHIC ROCKS In terms of their appearance, low grade metamorphic rocks look like their parent rock or protolith but with mineral alteration giving them a different colour (like greenschist). However, high grade metamorphic rocks more closely resemble igneous rocks than sedimentary rocks. Both igneous and high grade metamorphic rocks tend to have crystalline textures, the difference being that metamorphic rocks develop their crystalline texture as existing minerals recrystallize to form new mineral phases without passing through the liquid state. Like igneous and sedimentary rocks, metamorphic rocks are classified on the basis of texture and composition. Two main groups of textures are recognized, foliated and nonfoliated. Foliation results from recrystallization and the growth of new minerals oriented in such a way as to produce a layered, or leafy effect. Non-foliated texture develops by recrystallization of rocks composed predominantly of one mineral, such as sandstone (quartz) or limestone (calcite). FOLIATION Foliation is a planar element in metamorphic rocks. It develops in response to directed stress and results in the preferred orientation of platy minerals such as micas, amphiboles, and others along lines of least stress. The direction of foliation is perpendicular to the direction of the stress that acted upon the rock. When foliation is defined by mica grains, it is called cleavage. Different types of foliation develop with increasing metamorphic grade: (a) Slatey Cleavage The tendency of some rocks to fracture along nearly perfect, flat parallel planes. Under low grade metamorphism, very fine-grained platy minerals (such as micas, talc, and chlorite) commonly develop in slate. This metamorphic feature is not to be confused with bedding planes, which are a sedimentary structure. (b) Phyllitic Texture A parallel (but wavy or wrinkled) foliation of fine-grained (occasionally medium grained) platy minerals (mainly micas, chlorite, and graphite), exhibiting a silky or metallic lustre. It is developed best in phyllite, which is the product of a relatively low grade of metamorphism. 43 Lab manual for GEOG1120 (c) Schistosity A parallel to subparallel foliation of medium to coarse grained platy minerals (mainly micas and chlorite). It is commonly developed in schists, which are the product of intermediate to high grades of metamorphism. (d) Gneissosity A parallel to subparallel foliation of medium to coarse grained minerals in alternating bands of different composition. Ferromagnesian minerals such as hornblende or pyroxene usually form the dark bands and quartz and feldspars usually form the light bands. Most gneisses are the product of intermediate to high grade metamorphism. NON-FOLIATED TEXTURES Non-foliated textures typically result from the growth of existing minerals to form a more interlocking structure. They do not contain parallel planes of minerals; however, they may have stretched fossils or other grains that formed in response to directed metamorphic stresses. Examples of metamorphic rocks having non-foliated textures are marble and quartzite. Metamorphic rocks, which contain mineral grains all of one size are said to exhibit an equigranular, or granoblastic, texture. The grains are evenly shaped (sugary) in texture and typically without foliation. Hornfelsic texture is a fine-grained sugary texture found in dark coloured hornfelses (rocks formed at the innermost part of contact metamorphic aureoles). OTHER TEXTURES (a) Porphyroblastic Texture Some metamorphic rocks contain large crystals (porphyroblasts) set in a finer-grained groundmass, analogous to phenocrysts in igneous porphyries. It may be present in foliated or non-foliated metamorphic rocks. Often the porphyroblasts are non-platy minerals such as garnet or magnetite. (b) Lineation Lineation may develop in non-foliated rocks. Pebbles and sand grains that were originally spherical may be stretched by the directed stress within the rock. Deformation of limestone will often produce streaks of organic debris. These linear features are not true foliation, though they often give a banded appearance to the rock. An example of this feature can be seen in a stretched-pebble conglomerate. 44 Lab manual for GEOG1120 (c) Folds and Crenulations Folds (bends) or crenulations (parallel sets of very tiny folds up to 1 cm long) are also a common feature of metamorphic rocks. Many gneisses contain these deformation features. COMPOSITION OF METAMORPHIC ROCKS Mineral Changes Various mineralogical changes occur in rocks as they are metamorphosed. The new minerals that appear represent a crystalline configuration that is more stable at the increased temperature and/or pressure. The most common changes we observe are: (a) Recrystallization Occurs when small crystals of one mineral slowly convert to fewer, larger crystals or the same mineral without melting of the rock. For example, microscopic muscovite crystals in a slate can recrystallize to a larger size (but still microscopic) in phyllite and an even larger size (macroscopic) in schist. These changes indicate increasing grade of metamorphism. (b) Neomorphism Is the process by which minerals not only recrystallize, but also form different minerals from the same chemical elements. In most, the atoms present in the parent rock are the only ones that metamorphism can rearrange to create new minerals. The resulting rock is therefore strongly controlled by the nature of the parent rock, or protolith. The mineral composition of a metamorphic rock depends on the composition of the parent material. Figure 20 illustrates the typical transition in mineralogy that occurs as a clay-rich mudstone undergoes progressive neomorphism, and the metamorphic rocks that result. A basal, which contains mostly mafic minerals, will produce a different suite of minerals during metamorphism than will the shale. Figure 20 Progressive metamorphism of a shale. 45 Lab manual for GEOG1120 Some minerals (quartz, feldspars, micas) that you have already seen in sedimentary and igneous rocks are common in many types of metamorphic rocks. Others are important indicator minerals because they reflect a specific chemical composition and/or intensity of metamorphism. Some minerals (staurolite, cordierite, andalusite, sillimanite, kyanite) originate only in metamorphic rocks. (a) Chlorite (Mg, Fe, Al)12[(Si, Al)8O20](OH)16 Various shades of green; common in metamorphosed basalts and Mg-Al-rich rocks; silky lustre; streak white or pale green; hardness = 2; commonly has micaceous cleavage; tabular crystals or scales (b) Garnet (alumino-silicates of Ca-Mg-Fe-Cr-Mn) Red, yellow, green; vitreous to translucent; hardness = 6.5-7.5; no cleavage, subconchoidal fracture; crystals are 12 sided or equidimensional (c) Staurolite Fe2Al9O6(SiO4)4(OOH)2 Red-brown, black-brown, yellow-brown; dull lustre; hardness = 7-7.5; one cleavage parallel to long axis, also conchoidal fracture; prismatic crystals (d) Kyanite Al2SiO5 [Andalusite-Kyanite-Sillimanite series] These three minerals or identical composition are stable at different pressure and temperature conditions. Kyanite and sillimanite are elongate minerals while andalusite tends to form shorter prismatic crystals. Kyanite is blue-green or blue-white; pearly, translucent to transparent; hardness = 4-7 (varies from face to face, can be scratched with a knife parallel to the length but not across it); bladed crystals Rock name Slate Phyllite Schist Gneiss Dominant minerals quartz, feldspar, mica, chlorite, clay quartz, feldspar, mica, chlorite chlorite, biotite, muscovite, garnet, hornblende, staurolite, kyanite, andalusite, sillimanite; porphyroblasts common, used in name quartz, feldspar in light coloured bands; dark bands may be hornblende, augite, garnet or biotite, kyanite, andalusite, sillimanite Texture/Structure very fine grained; has slatey cleaveage; dull sheen on cleavage; black, grey, red, purple, green fine grained, dense; phyllitic texture; conspicuous sparkly sheen due to mica fine to medium grained with directed or schistose texture Derived from mudstone, shale, tuff coarse grained with layered or gneissose texture; often blocky minerals lend it an “eyed” appearance, ie “augen gneiss” any rock; often plutonic igneous (granite) or sedimentary clastic mudstone, shale, tuff any rock; most commonly mudstone, shale, tuff Table 7 Foliated metamorphic rocks. 46 Lab manual for GEOG1120 Rock name Greenstone Dominant minerals chlorite, epidote, Na-plagioclase, ±calcite, ±actinolite, ±quartz quartz, ±mica, feldspars Texture/Structure Derived from aphanitic to fine grained; nondirected texture mafic to intermediate volcanic rocks (basalt, andesite); low grade metamorphism quartz sandstone or chert fine to medium grained; nondirected texture; involves recrystallization of existing quartz sand grains resulting in a more interlocking crystalline structure, schistose if mica present; difficult to tell grade calcite, fine to coarse grained; Marble dolomite, nondirected texture; ±mica schistose if mica present; difficult to tell grade original pebbles Stretched-pebble quartz, [feldspars, distinguishable, but conglomerate or strongly deformed Metaconglomerate micas] talc, amphibole fine to medium grained; Soapstone minerals soapy feel; white to grey carbon soft, dark grey, with Graphite greasy feel carbon bright, hard coal; breaks Anthracite Coal with conchoidal fracture; glassy lustre hornblende, medium to coarse Amphibolite plagioclase, grained; nondirected garnet, quartz texture; can be gneissose pyroxene, fine grained; grey, greyHornfels plagioclase green, black; result of contact metamorphism Table 8 Non-foliated metamorphic rocks. Quartzite limestone or dolostone conglomerate peridotite anthracite coal bituminous coal mafic igneous rocks basalt or gabbro 47