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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:




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

Website created: January 22, 1998
Website last updated: October 11, 1998