Download Rocks - Mid-Georgia Gem and Mineral Society

Rocks
Types:
1 ) Igneous Rock- formed when magma cools underground and crystallizes or
when it erupts onto the surface of the ground, cools and crystallizes.
Magma that erupts to the surface is called lava. When magma cools slowly
underground, the crystals are large enough to see. When it cools quickly
on the surface, the crystals are very small and you would need a
magnifier or microscope to see them. Sometimes, when the magma cools
quickly, if forms a kind of black glass you cannot see through.
Examples are: Obsidian, Granite, Rhyolite, Diorite, Basalt and Pumice.
2 ) Sedimentary Rock- forms from particles, called sediment, that are worn off other
rocks. The particles are sand, silt, and clay. Sand has the largest particles while clay has
the smallest. If there are a lot of pebbles mixed with the sand, it is called gravel. The
sediment gets turned into rock by being buried and compacted by pressure from the
weight above it. Another way it becomes rock is from being cemented together by
material that has been dissolved in water. Often, both cementing and compaction take
place together.
Examples are: Sandstone, Shale, Limestone, Conglomerate
3 ) Metamorphic Rock- is formed by great heat, or pressure, or both. The pressure can
come from being buried very deep in the earth's crust, or from the huge plates of the
earth's crust pushing against each other. The deeper below the surface of the earth, the
higher the temperature, so deep burial also means high temperatures. Another way that
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high temperatures occur is when magma rises through the earth's upper crust. It is very
hot and bakes the rock through which it moves. Hot liquids or gases from the magma also
can cause chemical changes in the rock around the magma.
Examples are: Slate, Marble, Gneiss, Phyllite, and Quartzite.
By understanding the three types of rocks, we can understand the never ending cycle of
the life of rocks. This is known as The Rock Cycle.
Rocks do not last forever. The weather, running water, and ice wear them down. All
kinds of rocks become sediment. Sediment is sand, silt, or clay. As the sediment is buried
it is compressed and material dissolved in water cements it together to make it into
sedimentary rock. If a great amount of pressure is exerted on the sedimentary rock, or it is
heated, it may turn into a metamorphic rock. If rocks are buried deep enough, they melt.
When the rock material is molten, it is called a magma. If the magma moves upward
toward the surface it cools and crystallizes to form igneous rocks.
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Minerals are essential in the making up of the rocks we see. Many of the rocks are just
metamorphic or sedimentary mixes of minerals. Granite is a mixture of quartz, feldspar,
and mica. You cannot have rocks without minerals but you can have minerals without
having rocks.
Mineral Identification
Streak: Streak is the color of a crushed mineral's powder. The color of a mineral's
powder may differ from the actual color of the mineral. This property can be useful for
mineral identification.
Almost every mineral has an inherent streak color, no matter what color the actual
mineral is. For example, Calcite occurs in many different colors, shapes, and varieties.
But every single variety of Calcite has a white streak. A streak is useful in distinguishing
two minerals with the same color but different streak. A good example is distinguishing
Gold (yellow streak), and Chalcopyrite (black streak).
Most light colored, non-metallic minerals have a white or colorless streak, as do most
silicates, carbonates, and most transparent minerals. The streak test is most useful for
identifying dark colored minerals, especially metals.
When testing for streak, the mineral must be crushed to determine the color of its powder.
The color of the powder is the color of the streak. Instead of actually crushing a mineral
to determine the streak, it is much simpler to swipe the mineral across a streak plate. A
streak plate is an unglazed piece of porcelain, such as the underside of a ceramic tile. This
is the most popular method of streak testing, since the color of the streak plate is white,
the color of the mineral trace is easy to see. For minerals that are harder than the streak
plate, this test cannot be used, since the mineral will remove tile material. This is rarely a
problem, though, since most minerals where this test will be significant are softer than the
streak plate (the streak plate has an average hardness of about 6½ on the Mohs scale).
Luster: Luster describes how a mineral appears to reflects light, and how brilliant or dull
the mineral is. The terms used to describe luster are:
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Adamantine, having the hard, sparkly look of a diamond
Glassy/Vitreous, having the look of glass;
Resinous, having the look of amber – not quite glassy;
Pearly, having the iridescent look of mother-of-pearl (though usually just barely);
Greasy/Oily, having the look of an oil-coated substance;
Silky, having the look of silk, fine parallel fibers of mineral – such as chrysotile
"asbestos;"
Dull, having a plain looking surface that is not submetallic.
Earthy, having the look of soil or clay.
Metallic, having the look of a polished metal.
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Submetallic, having the look of a metal that is dulled by weathering or corrosion.
Non-metallic, not looking like a metal at all.
Pitchy (also known as pitchlike)
Waxy (also known as waxlike)
Adamantine - Transparent to translucent minerals with a high refractive index yield an
adamantine luster, meaning they display extraordinary brilliance and shine. Examples
being diamond, cerussite and Cubic zirconia. Minerals with a lesser, but still relatively
high degree of luster are referred to as subadamantine, with some examples being garnet
and corundum.
Glassy/Vitreous - minerals have the luster of glass. (The term is derived from the Latin
for glass, vitrum.) This type of luster is one of the most commonly seen, and occurs in
transparent or translucent minerals with relatively low refractive indices. Common
examples include calcite, quartz, topaz, beryl, tourmaline and fluorite, among others.
Resinous - Resinous minerals have the appearance of resin, chewing gum or (smooth
surfaced) plastic. A principal example is amber, which is a form of fossilized resin.
Pearly - Describes a luster similar to the inside of a mollusk shell or shirt button. Many
micas have a pearly luster, and some minerals with a pearly luster have an iridescent hue.
Some minerals may exhibit a pearly luster on cleaved crystal surfaces parallel and below
the reflecting surface of a mineral.
Greasy/Oily - Minerals resemble fat or grease. A greasy luster often occurs in minerals
containing a great abundance of microscopic inclusions, with examples including opal
and cordierite. Many minerals with a greasy luster also feel greasy to the touch.
Silky - Minerals have a parallel arrangement of extremely fine fibers, giving them a
luster reminiscent of silk. Examples include asbestos, ulexite and the satin spar, a variety
of gypsum. A fibrous luster is similar, but has a coarser texture.
Dull - Minerals exhibit little to no luster, due to coarse granulations which scatter light in
all directions. An example is kaolinite. A distinction is sometimes drawn between dull
minerals and earthy minerals, with the latter being coarser, and having even less lustre.
Earthy - Having the look of soil or clay. Examples are limonite, iron ore, hematite, etc....
Metallic - Minerals with a metallic luster are opaque and reflective, like metal. The
metallic elements, most sulfides, and some oxides belong in this category.
Submetallic - Describes a mineral that is opaque to nearly opaque and reflects well. Thin
splinters or sections of submetallic minerals are translucent.
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Vitreous - This luster accounts for roughly 70 percent of all minerals. Minerals with a
vitreous luster have reflective properties similar to glass. Most of the silicates,
carbonates, phosphates, sulfates, halides, and hydroxides have a vitreous luster.
Resinous - This is the luster of many yellow, dark orange, or brown minerals with
moderately high refractive indices - honey like, but not necessarily the same color.
Greasy - Luster of a mineral that appears as if it were coated with grease.
Pitchy - Minerals with a tar-like appearance have a pitchy luster. Minerals with a pitchy
luster are usually radioactive and have gone through the process of metamiction.
Metamiction is where the crystal is degraded by the presence of radiation to where it
becomes an amorphous shape.
Waxy - A waxy luster describes a mineral that appears as if it were coated with a layer
wax.
Dull - This luster defines minerals with poor reflective qualities, much like unglazed
porcelain. Most minerals with a dull luster have a rough or porous surface.
Every mineral has a characteristic luster, but some minerals may have a different luster
on different specimens. There is no scientific method to determine luster. Often,
determining the luster of a particular specimen is personal; to some it may appear as one
type of luster, and to others as a different type.
Observe the specimen in well lit conditions where its luster is visible. The surface being
viewed should not be tarnished, unclean, discolored, or coated. Some minerals exhibit a
pearly luster on cleaved surfaces, so it is a good idea to check for luster on uncleaved
portions of the crystal.
Cleavage: In mineral terms, cleavage describes how a crystal breaks when subject to
stress on a particular plane. If part of a crystal breaks due to stress and the broken piece
retains a smooth plane or crystal shape, the mineral has cleavage. A mineral that never
produces any crystallized fragments when broken off has no cleavage.
Quality of cleavage can be categorized into five qualities:
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Perfect
Good
Poor
Indiscernible (Indistinct)
None
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Minerals with perfect cleavage will cleave without leaving any rough surfaces; a full,
smooth plane is formed where the crystal broke. Minerals with good cleavage also leave
smooth surfaces, but often leave over minor residual rough surfaces. On minerals with
poor cleavage, the smooth crystal edge is not very visible, since the rough surface is
dominant. If a mineral exhibits cleavage, but it so poor that it is hardly noticeable, it has
"indiscernible" cleavage. Minerals with no cleavage never exhibit any cleavage, thus
broken surfaces are fractured and rough.
Categorization of cleavage qualities is not scientifically affirmed. The above
categorization is used by most mineral references, but some guides categorize cleavage in
three or four groups, and may give them different names, such as "excellent" and
"distinct".
Many minerals exhibit cleavage only on one side, and some may exhibit different quality
cleavage on different crystal sides. The following criteria may be expected when
analyzing the cleavage of any particular mineral:
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One Direction
Two Directions
Three Directions
All Directions
These identify how many "directions", or planes, the crystal is exhibiting the cleavage on.
Each direction signifies the two opposite sides of a three-dimensional figure, (since
opposite sides will always exhibit the same cleavage properties). If a mineral has
cleavage in three directions, then every side of the mineral has cleavage (i.e. length,
width, and height). If a mineral occurs in modified crystals with more than six sides (i.e.
an octahedron) and exhibits cleavage on all the sides, than it has cleavage in "all
directions".
Combining the cleavage level together with the number of sides will measure the
cleavage of a mineral. For example, if a mineral has Good Cleavage, Two Directions,
this means that it has good cleavage on four out of six sides (while the other two sides
exhibit no cleavage). If a mineral has Perfect Cleavage, One Direction; Poor Cleavage,
Two Directions, this means that the mineral has perfect cleavage on two sides, and poor
cleavage on the other four.
In this guide, cleavage quality is measured in numbers, then the amount of sides,
separated by a comma. 1 is perfect cleavage, 2 is good cleavage, and 3 is poor cleavage.
If the cleavage of a mineral is written as 1,2 the mineral has perfect cleavage in two
directions. If all sides of mineral have the same cleavage, and the mineral often occurs in
modified crystals with more than six sides, than All Sides is written instead of a number.
If a mineral exhibits different cleavage on different crystal planes, there will be two
cleavage indicators separated by a semi-colon (;). For example, if the cleavage of a
mineral is written as 1,2;3,1, than it has perfect cleavage in two directions, and poor
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cleavage in one other direction. If a mineral exhibits indistinct or no cleavage,
Indiscernible or None is written in the cleavage field.
Cleavage Habit:
Different habits of cleavage exist on different minerals, depending on their mode of
crystallization. These forms of cleavage are:
Basal cleavage:
Cleavage exhibited on a horizontal plane of the mineral by way of its base. Minerals with
basal cleavage can sometimes be "peeled".
An example of basal cleavage are the mica minerals.
Cubic cleavage:
Cleavage exhibited on minerals of the isometric crystal system that are crystallized as
cubes. In this method of cleavage, small cubes evenly break off of an existing cube.
An example is Galena.
Octahedral cleavage:
Cleavage exhibited on minerals of the isometric crystal system that are crystallized as
octahedrons. In this method of cleavage, flat, triangular "wedges" peel off of an existing
octahedron.
An example is Fluorite.
Prismatic cleavage:
Cleavage exhibited on some prismatic minerals in which a crystal cleaves as thin,
vertical, prismatic crystals off of the original prism.
An example is Aegirine.
Pinicoidal cleavage:
Cleavage exhibited on some prismatic and tabular minerals in which a crystal cleaves on
the pinacoidal plane, which is the third dimension aside from the basal and prismatic
sides.
An example is Barite.
Rhombohedral cleavage
Cleavage exhibited on minerals crystallizing in the hexagonal crystal system as
rhombohedrons, in which small rhombohedrons break off of the existing rhombohedron.
An example is Calcite.
Fracture: Fracture is the characteristic mark left when a mineral chips or breaks.
Cleavage and fracture differ in that cleavage is the break of a crystal face where a new
face (resulting in a smooth plane) is formed, whereas fracture is the "chipping" shape of a
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mineral. All minerals exhibit a fracture, even those that exhibit cleavage. If a mineral
with cleavage is chipped a certain way, it will fracture rather than cleave.
There are several terms to describe the various mineral fractures:
Conchoidal - (kaNG' koidl) Fracture resembling a semicircular shell, with a smooth,
curved surface. An example of conchoidal fracture can be seen in broken glass. (This
fracture is also known as "shelly" in some reference guides.) i.e.: flint, quartz. Even
though it is an igneous rock, obsidian does illustrate this fracture well.
Uneven - Fracture that leaves a rough or irregular surface. i.e.: arsenopyrite, pyrite,
magnetite
Hackly - Fracture that resembles broken metal, with rough, jagged, points. True metals
exhibit this fracture. (This fracture is also known as "jagged".) i.e.: native metals such as
copper and silver
Splintery - Fracture that forms elongated splinters. All fibrous minerals fall into this
category. i.e.: kyanite
Earthy or crumbly - Fracture of minerals that crumble when broken. i.e.: limonite,
kaolinite, aluminite.
Even or smooth - Fracture that forms a smooth surface.
Subconchoidal - Fracture that falls somewhere between conchoidal and even; smooth
with irregular rounded corners. i.e.: fluorite
Some references may describe additional fractures not mentioned above, but those terms
are either synonymous or simply used as a verbal depiction of the authors inference.
Almost all minerals have a characteristic fracture. Some minerals of the same species
may exhibit a different fracture, but this is rare.
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Crystal Systems: The primary method of classification of crystals. The crystal system classifies
crystals into six distinct groups. They are:
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Isometric
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Tetragonal
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Hexagonal (which includes Trigonal)
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Orthohombic
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Monoclinic
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Triclinic
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Specific Gravity: is the dimensionless unit defined as the ratio of the density of a
substance to the density of water at 4° C (39° F).
Easy way to determine the specific gravity at home is;
1) dry weigh the specimen on a scale. A 300 gram scale should suffice.
2) put a plastic container on the scale and put just enough water in it to cover the
specimen.
3) zero tare the scale with the plastic container and water
4) suspend the specimen in the water without it touching the bottom or sides and
record the weight.
5) divide the first reading by the second and this will give you the ratio which is
your specific gravity.
Specimen of lead:
Dry weight (DW): 705 grams
Wet Weight (WW): 642.6 grams
705 - 642.6 = 62.4
705 ÷ 62.4 = 11.3 specific gravity
Difference (DW-WW): 62.4 grams
Specific gravity = 705 divided by 62.4 = 11.3 specific gravity
Acid test: Some rocks and minerals are carbonate compounds that dissolve rapidly in
acid, producing bubbles and/or fizz. The test is done by the application of a drop or two
of acid to the stone. The bubbles or fizz is indicative to carbonates being present in the
specimen.
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Hardness: In 1812 the Mohs scale of mineral hardness was devised by the German
mineralogist Frederich Mohs (1773-1839), who selected the ten minerals because they
were common or readily available. This is referred to as The Mohs Scale of Hardness.
This is a test to see if a rock or mineral can be scratched and at what point does it do so.
Hardness
Mineral
Associations and Uses
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1
2
Talc
Gypsum
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2.5
3
3
4
4- 4.5
4- 5
5
5.5
6
6.5
6.5
6- 7
7
7+
8
8.5
9
9- 10
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Fingernail
Calcite
Limestone and most shells contain calcite.
Copper Penny
Fluorite
Fluorite from fluorite is used to prevent tooth decay.
Platinum
Iron nail
Apatite
Apatite is a mineral in vertebrate bones and teeth.
Knife Blade
Orthoclase
Orthoclase is a feldspar.
Iron Pyrite
Masonry or Steel nail
Glass
Quartz
Most common mineral in the Earth's crust
Hardened steel file
Topaz
A variety of beryl.
Masonry drill bit
Corundum
Sapphire and ruby, twice as hard as topaz.
Wurtzite Boron Nitride
Diamond
Used in jewelry and cutting tools. Four times harder
than corundum.
Talcum powder
Plaster of Paris. Formed when seawater evaporates
from the Earth's surface
The basic test kit for hardness at home is your fingernail, a nail, small glass plate, a small
knife blade, a steel file, copper penny and a steel or masonry nail.
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A good test kit for testing rocks, minerals and fossils should contain the following;
1) Unglazed ceramic tile: the back side of a white glazed tile can be used.
2) Small pane of glass: either have the edges sanded or taped to reduce cuts. 6-7
Mohs scale
3) Small dropper bottle with acid solution: ( I use a solution of sodium bisulfate and
water to create a mild sulfuric acid but hydrochloric acid can be used.)
4) Copper penny: 3 Mohs scale
5) Knife blade: Good pocket knife with a small blade.
6) An iron nail: 4- 5 Mohs scale
7) Steel or masonry nail: 6.5 Mohs scale
8) Flat iron file: most are already hardened: 7+ Mohs scale
9) Masonry drill bit: 8.5 Mohs scale
10) Small magnifier or loupe
11) Magnet: to test for iron
12) Good pocket guide to Rocks & Minerals. I have the Audubon Guide to Rocks and
Minerals, Roadside Geology of Georgia, and Minerals of Georgia: Their
Properties and Occurrences.
Other phrases and conditions to be aware of;
Pseudomorph: The name means "false form". It is where a mineral or mineral compound
that appears in an atypical form, resulting from a substitution process in which the
appearance and dimensions remain constant, but the original mineral is replaced by
another.
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