Igneous Rocks

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Igneous
Rocks!
By Teach 1
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Igneous Rocks come in three basic varieties,
mafic, intermediate, and felsic. Ultramafic rocks
are lower in silica than even mafic rocks.
Ultramafic Having very low silica content and
very rich in Fe and Mg
Mafic Mafic is used for silicate minerals,
magmas, and rocks which are relatively high in
the heavier elements. The term is derived from
using the MA from magnesium and the FIC from the
Latin word for iron, but mafic magmas also are
relatively enriched in calcium and sodium.
Intermediate Igneous Rocks that have a chemistry
between mafic and felsic (silica amounts between
53 to 65 ).
Felsic Silicate minerals, magmas, and rocks
which have a lower percentage of the heavier
elements, and are correspondingly enriched in the
lighter elements, such as silica and oxygen,
aluminum, and potassium. The term comes from FEL
for feldspar (in this case the potassium-rich
variety) and SIC, which indicates the higher
percentage of silica.
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Muscovite Mica insulation, electronics, filler in
plasterboard, cement, paint peels in thin sheets
Biotite Mica insulation, electronics, filler in
plasterboard, cement, paint, peels in thin sheets
Calcite the matrix in cement (CaCO3), optical
equipment (3 on Mohs scale of hardness)
Sulfur matches, fireworks, medicine(sulfa
drugs), sulfuric acid,vulcanization of rubber
yellowcolor, flammable, smells acrid
Quartz glass, crystal, radios, watches,
computers, electronics, jewelry (onyx, agate,
amethyst) (7 on Mohs scale)
Graphite pencil lead, dry lubricant,batteries
(hardness of 2)
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Gypsum drywall, plaster of Paris(2 on Mohs
scale)
Orthoclase Feldspar abrasives, filler in paint
andplastics most commonminerals in igneous
rocks
Plagioclase Feldspar abrasives, filler in paint
andplastics most commonminerals in igneous
rocks
Hematite iron ore, cabochon jewelry(when
polished)
Galena lead ore for pipes, xrayshielding,
fishing weightsvery dense
Halite table salt, manufacture ofsoap, paper,
petroleum glass tastes salty
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Magnetite iron ore magnetic
Talc talcum powder, paints,ceramics, paper
coatingssoftest mineral(1 on Mohs scale)
Olivine source of peridotite forjewelry green
color
Pyrite sulfuric acid productionknown as fools
gold
Limonite iron ore, yellow pigment forpaint
adds color to soil
Barite barium ore for gastrointestinalx-rays,
white pigment for paint
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Hornblende common mineral inigneous rocks
Dolomite lime for neutralizing acidic soil,road
aggregate, building stone
Bauxite aluminum ore for soft drinkcans,
softball bats, alloywheels, lawn furniture
Copper coins, pipes, wire, cookingutensils,
jewelry ductile,malleable and conductive
Kaolinite ceramics, china, pottery,filler in
paper
Chalcedony arrowheads, drivewaygravel,
ornamental stone,cabochon jewelry
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Augite (pyroxene) is only of importance to
collectors, scientists and petrologists
Hypersthene As mineral specimensand there is an
ornamental variety
Diopside has no industrial use, good specimens
can be used as gemstones
Amphibole semiprecious gemstones as asbestos
fire proofing materials, cement, brake pads,
plastics, paper products and textiles
Kimberlite diamond ore. Diamonds used for
jewelry, cutting tools, polishing hard metal,
bearings for laboratory instruments
Ilmenite As the major ore of titanium, a minor
ore of iron, as a flux in blast furnaces, as an
abrasive
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Feldspar
Feldspar is the most common rock-forming mineral
(about 60 of the earths crust). The mineral
name feldspar is derived from the German words
feld spar. The word "feld" is "field" in German
and "spar" is a term for light colored minerals
that break with a smooth surface. Feldspar
minerals are usually white or very light in
color, have a hardness of 6 on the Mohs Scale of
Hardness and perfect to good cleavage (plane of
breakage) in two directions. Feldspar is a
common name that applies to a group of minerals
with a general chemical formula of x
Al(Al,Si)3O8, where x can be sodium (Na) and/or
calcium (Ca) and/or potassium (K).
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Why is feldspar important? The feldspars are a
family of silicate minerals which occur in
igneous rocks. There are many different members
to the feldspar group. Obviously, silica and
oxygen form the foundation for the group, but
calcium, sodium, and potassium are also present.
One of these elements is usually dominant, but
most of the feldspars contain all 3 in varying
amounts. It is the proportions of these 3
elements which help determine which specific
feldspar is formed. The feldspars are divided
into 2 broad categories plagioclase, which
contains calcium and sodium and orthoclase,
which contains potassium.
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Plagioclase and Orthoclase It is important to be
able to distinguish between plagioclase and
orthoclase.. You can make an educated guess in
the field based on several assumptions, and a few
easily identifiable physical features - color and
the presence (or absence) of "striations."
Striations are often visible on plagioclase, and
look like very fine (almost microscopic) parallel
lines cut into the face of a mineral fragment.
All feldspars which have striations are
plagioclase, but not all plagioclase has
striations. Only orthoclase can be pink, and only
plagioclase can be dark gray to black.
Unfortunately, both can be light in color, and
since most feldspars are nearly white we're often
still faced with a nasty identification problem.
If it's white and has striations, I call it
plagioclase. If it's white but you can't see any
striations, just call it feldspar. The following
graphic summarizes the field identification of
feldspar
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Basalt
A fine-grained, dark-colored rock of volcanic
origin composed primarily of plagioclase
feldspar, and pyroxene, together with other
minerals, usually including olivine and ilmenite
(an oxide of iron and titanium). Basalt is the
most common extrusive igneous rock on the
terrestrial planets and covers about 70 of
Earths surface.
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Pyroxene
Any of a group of dark, dense, rock-forming
silicate minerals rich in calcium, iron, and
magnesium and commonly found in basalt. The group
includes augite, hypersthene, and diopside, with
composition varying as a mixture of FeSiO3,
MgSiO3, and CaSiO3. Pyroxenite is an igneous rock
composed largely of pyroxene.
Augite
Hypersthene
Diopside
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Amphibole
Amphibole is a name given to silicate minerals
with the general composition Ca2(Fe,Mg)5Si8O22(OH)
2. The commonest form is hornblende other
species include anthophyllite, cummingtonite,
tremolite, actinolite, riebeckite, and
glaucophane. A variety of jade, called nephrite,
consists of actinolite in a finely fibrous form.
Hornblende
Nephrite (actinolite)
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Gabbro
Gabbro is a dark, coarse-grained, intrusive
igneous rock chemically equivalent to basalt. It
is a plutonic rock, formed when molten magma is
trapped beneath the Earth's surface and cools
slowly into a hard, coarsely crystalline mass. It
is dense, greenish or dark-colored and contains
varied percentages of plagioclase feldspar,
pyroxene, amphibole, and olivine (called olivine
gabbro when olivine is present in large
quantities). Quartz gabbros are also known to
occur and are probably derived from magma that
was oversaturated with silica.
Plutonic rocks (also called intrusive igneous
rocks) are those that have solidified below
ground.
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Pegmatites
Londonite (Madagascar)
Molybdenite
It is an igneous rock with extremely coarse grain
size. To elaborate, a pegmatite has the same base
constituents as granite (quartz, feldspar, mica)
except the crystals are larger in size. In basic
granite, the rock forming minerals usually
crystallize in sizes between 0.4 and 1 inch. In
pegmatites, the minerals can crystallize into
larger sizes. It is not uncommon to find crystals
over a meter in length in larger pegmatites.
Large pegmatites may extend from 5 to 100 feet
thick and 100 to 1000 feet in length. Crystals
can grow to tremendous sizes, such as quartz
crystals 17 feet long and 8 feet in diameter,
orthoclase crystals 33 feet by 33 feet, beryl 19
or more feet in length, tourmaline crystals 10
feet long and mica sheets with up to 68 square
feet of surface area.
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Kimberlite
Kimberlite is a mica peridotite which occurs at
Kimberley, South Africa, the source of rich
deposits of diamonds. These diamonds were
originally found in decomposed kimberlite which
was colored yellow by limonite, and so was called
"yellow ground." Deeper workings encountered less
altered rock, undecomposed kimberlite, which
miners call "blue ground." Kimberlite occurs in
"kimberlite pipes," vertical columns of rock that
rise from deep magma reservoirs.
Peridotite is a dense, coarse grained ultrabasic
rock, consisting mainly of the minerals olivine
and pyroxene
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Porphyritic Texture
Many rocks with an overall fine-grained texture
display scattered minerals that are clearly
greater than 1 mm across. This combination of two
crystal size populations is called porphyritic.
It indicates that the magma sat and cooled a bit
below the Earth's surface, thus giving time for
the large crystals to grow, and then it erupted
onto the surface, which allowed the rest of the
rock to cool very quickly. Porphyritic textures
thus indicate two-stage cooling histories.
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The End