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Rock or Mineral? • It is not easy to tell the difference between rocks & minerals because there are so many kinds of them. It takes years of study to be able to accurately identify a mystery rock & where the specimen came from. • All rocks are made of 2 or more minerals, but minerals are not made of rocks. MINERALS A mineral is the same all the way through. It is a sample or a specimen rather than a rock. There are about 3000 known minerals on earth. Minerals: Building Blocks Minerals are born of fluid, heat, and pressure. They are forged underground, where forces that have been at work for billions of years continue to make more minerals. A mineral is a naturally occurring, inorganic (nonliving) solid having a specific chemical composition. Chemical Formula Every mineral has a unique arrangement of atoms that determines the mineral type. All minerals have a chemical formula, which is an analysis of the types and amounts of elements present in a mineral. The chemical formula of the mineral hematite is Fe2O3. The letters describe the element type (Fe = iron, O = oxygen), and the subscripted numbers describe the amount of those atoms in each molecule. A Hematite molecule has 2 iron (Fe) atoms and 3 oxygen (O) atoms. Classes of Minerals The Carbonates The Silicates The Sulfides The Oxides Quartz Family The Halides The Sulfates Elements http://www.rocksandminerals4u.com/minerals.html Physical Properties most useful for mineral identification: • • • • Cleavage Fracture Hardness Specific Gravity • Streak • Associated Minerals • Notable Localities •Color •Luster •Transparency •Twinning •Crystal Systems •Technical Crystal Habits •Descriptive Crystal Habits PROPERTIES OF MINERALS These are the most common characteristics used when describing minerals. • Color – this varies depending on the chemicals present and is the least informative in identifying a mineral variety • Luster – what the surface looks like in the light • Specific Gravity – how heavy it feels, heft • Crystal Form – shape of crystal, shape the mineral would take if it had room to grow in a cavity, not massive – some minerals have a number of different crystal shapes • Cleavage – pattern when mineral is broken –planes or conchoidal fracture • Tenacity - toughness, how cohesive the mineral is, if it falls apart • Hardness – what it can scratch & what scratches it • Transparency - The ability to transmit light. Many rocks that are opaque when in a chunk, are translucent when cut into very thin slices. Gems stones are often valued on how transparent they are. • Special Properties– magnetism, chatoyancy (cat’s eye), fluorescence, odor, streak, burn test, conductivity, double refraction, & radioactivity. COLOR • Color is one of the physical properties most commonly used to describe minerals, but it is not a very good property to use t identify minerals. • Some minerals are nearly always the same color like azurite (blue) and sulfur (yellow) • Many minerals come in a variety of colors – the changes are caused by chemical impurities or through exposure to heat • Color can change when the surface is exposed (tarnishes or oxidizes) • Some minerals have common names (varietal names) that describe a specimen with a certain color eg. Quartz – rock crystal (colorless), smoky quartz (brown), citrine (yellow), amethyst (violet), rose quartz (pink) • Color can be described as metallic or non-metallic and is often described along with luster though different. • Rocks are often distinctive or named because of a certain color which occurs because of their mineral content. Color Descriptions Metallic or Nonmetallic Intensity of colors: Dark very dark light, pale deep dull shiny bright Color is distributed: streaked splotchy mottled speckled layered banded Other words that describe luster: shiny, sparkly shimmering, opalescent frosted, milky Words LUSTER - describes the way light reflects off of the surface of a mineral: Description Example dull / earthy very dull, mainly in minerals that are porous kaolinite, orthoclase waxy opal, chalcedony like the surface of a candle greasy / oily nepheline pearly like a pearl, play of colors, light silky has a shiny surface like a piece of silk cloth glassy / vitreous resinous adamantine talc, muscovite mica gypsum, kernite, ulexit & in fibrous minerals looks like glass quartz, rock-forming minerals, obsidian looks like freshly-broken shellac, usually yellow- sphalerite brown high luster, almost brilliant, "diamond-like" sphalerite sub-metallic silvery or metallic luster but mineral is transparent or translucent metallic very shiny, like processed metals, highly reflective, opaque minerals hematite pyrite, gold, silver SPECIFIC GRAVITY •Specific Gravity (SG) indicates how many times more the mineral weighs compared to an equal amount of water (SG 1). • So if you have a bucket of silver, it would weigh 10 times as much as a bucket of water. •If you have a bucket of calcite, it would only weigh about 2 1/2 times as much as a bucket of water. •That is why we think of metals as being "heavy“ or "heft" of an object. •They are heavy compared to other things that we are used to picking up. Description very light light SG Mineral Examples <2 borax 2 - 2.5 gypsum, halite, selenite, ulexite average 2 – 3 calcite, dolomite, feldspar, muscovite mica, quartz, talc, turquoise, above average / 3 - 4 biotite mica slightly heavy heavy 4 – 5 almandine garnet, apatite, barite, celestite, fluorite very heavy 5 – 10 galena, hematite, magnetite, nickel-iron, pyrite extremely heavy> 10 gold, silver super heavy 20+ platinum TENACITY Tenacity is how tough or how easily a mineral will break, split, crumble or change shape. •elastic ~ can be bent & resume previous shape ~ mica •ductile ~ can be pulled to make thin threads ~ gold •flexible •fragile ~ break into pieces easily •friable ~ crumbles easily •malleable ~ flatten into thin sheets w/o break ~ gold •sectile ~ can be cut for shavings ~ gypsum •tough CLEAVAGE Cleavage is when a mineral breaks with smooth flat surfaces. Cleavage can be described as perfect, good, imperfect, poor. It can also be described as: •Perfect 1 way ~ breaks on one perfect cleavage plane, crystals break into slices, sheets peel off •Perfect 2 ways ~ breaks into elongated boxy shapes, 90 degree angles •Perfect 3 ways ~ breaks into perfect rhombs, pieces look like squished boxes •No cleavage ~ does not break regularly FRACTURE Fracture is when a mineral breaks, but the surface is not regular, does not show cleavage. conchoidal ~ curved break like what happens with thick glass or bottle bottom, shell shaped, can be rough or smooth •jagged ~ metals, sharp point that scratches or snags fingertips, hackly •splintery ~ fibrous •uneven ~ rough surface, not smooth TRANSPARENCY ~ Transmitting Light Through Minerals transparent •clear, see right through it when it is celestite, quartz (rock sliced thin crystal), •called "gemmy", desirable for gemstones selenite translucent •see shapes & shadows through it when it calcite, quartz, sphalerite is sliced thin •chemical impurities can cause the mineral to be cloudy opaque •can’t see light through it at all when it is metals, gypsum sliced thin •rarely used for gemstones STREAK ~ powdered residue left on a black or white tile. Not necessarily the same color as the mineral. HARDNESS Friedrich MOH’S SCALE OF HARDNESS Scratching tools: fingernail (2.2) copper penny (3.5) pocket knife or common nail (5.2) piece of glass (5.5) steel file or concrete nail (7.5) piece of corundum (9) Moh’s Scale Rating 1 Very Soft 2 Soft Description Easily crumbles. Can be scratched with a fingernail (2.2) Can be scratched with a fingernail (2.2) 3 Soft Can be scratched with a copper penny (3.5) Gypsum, Soapstone Calcite 4 Semi-Hard Can be scratched with a common nail (5.2) Fluorite 5 Hard Can be scratched with a common nail (5.2). Apatite 6 Hard N. B. Mineral of hardness 6 or more will scratch glass. Can be scratched with a concrete nail (7.5). Feldspar 7 Very Hard 8 Very Hard Mineral Example Talc Quartz Topaz 9 Extremely Used in industrial tools for cutting, grinding & Corundum Hard sanding. 10 The Hardest Diamond is used to cut all minerals including Diamond diamonds. Notes for testing: Each mineral can scratch the minerals with lower hardness ratings. Each mineral can scratch itself. Don’t press hard, normal scratching should do. Weathered surfaces are softer. Corners or edges of crystals are softer. Small pieces seem softer than large pieces. When you scratch, take a close look at the scratch line - which often looks white. Is it really a scratch or is it a powder line made from the tool you used because it was softer than the item you were trying to scratch? Crystalllographic Systems •Minerals usually form distinct crystals and their shape plays an important part in their identification. • The study of crystals is called crystallography and includes the study of natural crystal but crystals formed by metal alloys, chemicals, and other synthetic materials. •Specific tools, such as an x-ray spectrometer, are used to find and distinguish new minerals and verify the identification of specimens by the arrangement of atoms and/or ions. Crystals: The Form Most minerals occur naturally as crystals. Every crystal has an orderly, internal pattern of atoms, repeating that pattern. Symmetry is a regular, repeated pattern of component parts. The shape of the resulting crystal mirrors the internal arrangement of the atoms. As crystals grow, differences in temperature and chemical composition cause variations. Crystals need ideal growing conditions and room to grow or they mesh together to form a conglomerated mass like most rocks. The museum-quality specimens shown in the images here, grew in roomy environments that allowed the geometric shapes to form. Crystals Grouped by Properties There are four main categories of crystals, as grouped by their chemical and physical properties: Covalent Crystals Many covalent crystals have extremely high melting points. Examples :diamond and zinc sulfide crystals. Metallic Crystals Individual metal atoms of metallic crystals sit on lattice sites Metallic crystals are very dense and have high melting points Ionic Crystals Ionic crystals are hard and have relatively high melting points. Table salt (NaCl) is an example of this type of crystal. Molecular Crystals Molecular crystals tend to be soft with relatively low melting points. Rock candy is an example of a molecular crystal. The two most common methods of grouping crystals are according to their crystalline structure and their chemical/physical properties: Crystal Grouped by Lattices (Shape) There are seven crystal lattice systems. Cubic or Isometric - not always cube shaped! You'll also find octahedrons (eight faces) and dodecahedrons (10 faces). Tetragonal - similar to cubic crystals, but longer along one axis than the other, forming double pyramids and prisms. Orthorhombic - like tetragonal crystals except not square in cross section (when viewing the crystal on end), forming rhombic prisms or dipyramids (two pyramids stuck together). Hexagonal - six-sided prisms. When you look at the crystal on-end, the cross section is a hexagon. Trigonal - possess a single 3-fold axis of rotation instead of the 6fold axis of the hexagonal division. Triclinic - usually not symmetrical from one side to the other, which can lead to some fairly strange shapes. Monoclinic - like skewed tetragonal crystals, often forming prisms and double pyramids. http://www.inlandlapidary.com/user_area/systems.asp