THE ROCK FORMING MINERALS
What we will do is look at how to recognize the actual minerals,
we will start with the major minerals and then look at some of the
more common accessory minerals after that. We will take it by mineral
groups to help everyone keep things sorted out *G*. Most of the Rock
Forming Minerals are silicates so we will take them last in each
sequence }:)
so - the rock forming minerals }:)
I)
Carbonates
( no native elements, sulfides, or oxides in the major minerals)
there are really only 2 carbonate minerals we need to consider:
Calcite
(CaCO3)- generally white to gray but the color is variable
it is low density (2.7) and soft (3)the crystals are hexagonal
forming rhombohedral (off kilter rectangles), scalenohedral
(pyramidal forms) and prismatic (columns) in addition the mineral is
highly reactive to hydrochloric acid (HCl) and bubbles vigorously
when exposed to it when massive the softness and the effect of acid
are used to identify it in limestones and marbles and as an accessory
in other rocks in crystal form the shapes of the crystals and the
perfect cleavage are used.
Dolomite
- CaMg(CO3)2 - forms white to pink or yellow
rhombohedral crystals but can be variably colored in massive form. It
is slightly heavier than calcite ( 2.9) and slightly harder (3.5 - 4)
and most useful is the fact that it is only weakly reactive to cold
HCl but reacts vigorously in warm acid. Between them Calcite and
Dolomite make up over 95% of all Carbonates found.
II)
Sulfates:
Gypsum
(CaSO4*2H2O) is monoclinic (look it up in last
weeks notes }:)) and it forms generally clear crystals in massive
form it is a white to pinkish color it is a light (2.35) and soft (2)
mineral that is soluble in both HCl and H2O. One frequently collected
form is the desert rose - a grouping of pinkish tabular crystals, a
second collected form is known as alabaster. Due to its solubility in
water its normally found only in desert/dry regions and in some cases
it has been dried enough to have altered to a water free Sulfate
called
Anhydrite
III)
Halides:
Halite
(NaCl) - better known as salt. Halite forms cubic crystals that are
normally clear unless contaminated. It has a perfect cubic cleavage
and is both soft (2.5) and light (2.15). Its most distinguishing
features how ever are its ready solubility in water - and its salty
taste. Normally found only in desert regions or in subsurface
deposits and domes.
ok any questions on these 4 minerals before we go on?
{LD} Nope. I'm quite happy with the explanations.
Ok before we go on to the silicates let me add some of the uses for
each of these minerals.
Halite - dietary requirement, use to make HCl, NaOH, Na(sodium);
used as a preservative for foods
Gypsum - gypboard, plaster of Paris, pottery flux, fertilizer;
Alabaster - decorative stone carvings
Calcite and Dolomite: optical prisms, cement, Marble, Lithography,
fillers in rubber and paint industries (and in foods), chalk. Calcite
is also used in the making of CO2 gas.
{The-Y} Is CO2 bad gas? cover eyes or cover nose?
CO2 is carbon dioxide - the fire extinguisher gas. any
other questions?
Gypsum and Halite are normally found where lakes or seas dried up
leaving the dissolved minerals behind as layers. Carbonates form as
reefs and sands in shallow tropical to .subtropical seas.Also the
carbonates react to acid, the gypsum to acid and water, and the salt
to water.
The remaining Major rock forming minerals are all
silicates, and can be divided into 2 main groups - the dark minerals
(mafic) and the light minerals
(felsic) or we will look at them by structural
classes (also from last week }:))
Nesosilicates
(loose unattached tetrahedra (pyramids)):
1)
Olivine
( in fine crystal form it is the August birth stone Peridot and was
the Emerald of the ancients). The chemical formula is (Mg,
Fe)2SiO4 - actually it is a solid solution of 2
minerals Forsterite (Mg2SiO4) and Fayalite
(Fe2SiO4) with the relative percentage of
Forsterite increasing with the temperature of crystallization. As the
iron content increases the density increases from 3.25 to 4.20 and
the hardness from 6.5 to 7. It orthorhombic and generally is found as
sand sized grains and crystals in basalts and gabbros as well as in
peridotites. Forsterite rich rocks are used as bricks in high
temperature oven and as an ore of magnesium (Mg). Large clear
crystals are known as Peridot and normally faceted as gems.
Inosilicates:
(single or double chains of tetrahedra)
Pyroxenes:
there are actually 2 groups of pyroxenes - those that contain Calcium
and those that don't. Like olivine the pyroxenes are solid solutions
in which the amount of Iron (Fe) and Magnesium (Mg) varies depending
on the temperature of crystallization.
Calcium Pyroxenes range from
Diopside
(CaMgSi2O6) to
Hedenbergite
(CaFeSi2O6), and as the iron content increases
the temperature of crystallization drops while the density increases
from 3.2 to 3.6. The hardness is about 5.5 but can vary from 5 to 6.
The normal form found is called
Augite
and has the formula
(Ca,Na)(Mg,Fe,Ti)(Si,Al)2O6.
Non-calcium pyroxenes form a series from
enstatite
(Mg2Si2O6) to
Hypersthene
(Fe2Si2O6) and has similar density
and hardness to the other . Pyroxenes. Pyroxenes normally form blocky
crystals that have a dark color (blue, green, brown or black) and a
90 degree cleavage that is very prominent This cleavage is actually
the means of identifying pyroxene in the field and separating it from
the next group of minerals.
Amphiboles
((Ca,Na)2-3(Mg,Fe,Al)5(Al,Si)8O22(OH)2
- the most common of which is
Hornblende
- are somewhat lighter than the pyroxenes ( 3.0 - 3.2) but equally
hard and with the same colors. The primary means of identifying them
from the pyroxenes is by their cleavage angles ( and crystal face
angles). Pyroxenes always have a 90 degree angle (cubelike) while
Amphiboles have a diamond like cross section with cleavages at 60
degrees and 120 degrees.
The dark minerals form 4 solid solutions - olivine, pyroxene,
amphibole, and mica. The normal pyroxene is augite which is really
sort of a mid-point in the solution while the main amphibole is
hornblende - also the middle of the range. Amphibole and pyroxene
often look much alike but can be told apart by looking for the
cleavages or the angles of the crystal faces. Pyroxene will have 90
degree angles while amphibole will have 60/120 degree angles. That
make most of this easier so far?
* A. nods
Phyllosilicates
(sheet like):
the first are the
micas
: these form hexagonal books of mineral sheets. They are moderately
dense (2.8 - 3.2) but soft (2.5 to 3) the 2 main forms are:
Biotite
(K(mg,
Fe)3(AlSi)3O10(F,OH)2 - a
dark brown or green color
Muscovite
KAl2(AlSi)3O10(OH)2 which
is normally silvery in color
micas are often used to make electrical and heat insulation and
they are the mineral that normally gives a sparkle to rocks (or
toothpaste) The micas are easily recognized by their sheety form and
separated by color - dark is Biotite and light is muscovite.
the second group of phyllosilicates are actually too small to see
- the crystals are found in sediments more than rocks - these are the
clays.
The 3 most common types are Kaolinite (hydrous aluminum silicate),
Montmorillonite which contains additional calcium (Ca), sodium (Na),
and Magnesium (Mg); and Vermiculate which contains Magnesium and Iron
but no Calcium and sodium.
Tectosilicates
( 3-D networks):
Feldspars
- these are aluminum silicates with sodium, potasium and calcium.
They form two groups the potassium feldspars and the plagioclase
feldspars.
Potasium feldspars (KAlSi3O8) are:
Sanidine,
Orthoclase
Microcline
which form in order as the temperatures are decreasing.
Sanidine and Orthoclase are normally white tabular crystals or
masses
Microcline is generally pinkish with one variety
(amazonite/amazonstone) being greenish.
the potasium feldspars (K-spars) are normally found in granitic
(high silica) volcanic rocks or sedimentary rocks formed near such
sources.
Plagioclase feldspars form a solid solution from
anorthite
(CaAlSi3O8) to
Albite
(NaAlSi3O8). The greater the temperature of
crystallization the greater the Calcium percentage in the feldspar.
The feldspars are fairly light (Kspar = 2.6, Plag 2.6-2.8) and
moderately hard (6). They can be identified by their white to grey or
pink colors and the laths they normally form in the rocks.
Quartz.(SiO2)
- pure quartz forms clear hexagonal crystals with its density (2.65),
and hardness (7) staying constant despite color and form changes. It
is normally colorless to milky but can take on may colors if
.impurities are present
ok any questions on these?
{a} not so far DH
ok then why don't we call it a night at this point and we will
look at the Accessory Rock Forming
Minerals next week
{a} okie
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