The Common Rock-Making Minerals
Although many minerals are found as rock constituents, those which can be termed common and characteristic rock-making minerals are comparatively few in number. The following list gives the names of these minerals, with a brief statement in each case of the types of rocks in which they most commonly occur.
Quartz. Quartz, SiO2, is a very common and widely distributed rocks-making mineral. It is found in all the light-colored, acid igneous and metamorphic rocks. It is the chief constituent of sandstones and metamorphic rocks. It is the chief constituent of sandstones and quarzites. It is to be recognized by its hardness (7), its vitreous luster, lack of cleavage and conchoidal fracture. When it occurs in igneous rocks it often has gray or smoky color.
The Feldspars. The feldspars include orthoclase and microline, KALSi3O8, albite, NaAlSi3O8, amorthite, CaAl2Si2O8, and various mixtures of these last two as oligoclase (3 albite to 1 anorhite), andesine (1 albite o 1 anorthite) and labradorite (1 albite to 3 anorthite). They are very common rock-making minerals and are found in a great variety of rock types. They are characteristic of most igneous rocks, and frequently constitute a large proportion of them. They are found in the gneissses and to a less extent in some sandstones. They are to be distinguished by their two cleavages at right angles or so, their vitreous luster and their hardness (6). It frequently is difficult, if not impossible, to tell the kind of feldspar present in a rock by inspection alone. Under favorable conditions twinning strations may be observed on the best cleavage face. Which would indicate that the feldspar belonged to the plagioclase group and could not be orthoclase.
Nephelite. Nephelite is a silicate whose composition is essentially NaAlSiO4. It is restricted in its occurrence, being found only in certain igneous rocks, such as the nephelite syinites, which are low in percentages of silica. It is often mistaken for quartz, but the two minerals are practically never found together. It is best determined by a chemical test. Unlike most rock-making minerals, it is readily soluble in hydrochloric acid and the solution gelatinizes on evaporation.
Sodalite. Sodalite, Na4(AlCl)Al2(SiO4)3, is similar in its occurrence to nephelite, with which it is commonly associated. It may be greenish gray or white in color, but is usually blue. Hauynite and noselite are similar but rare species which occur in the same way.
Leucite. Leucite has the composition Kal (SiO3)3. it is a rare rock-making mineral found chiefly in rather basic lavas. It is commonly in the form of phenocysts which show trapesohedral forms. It is white to gray in color with a dull vitreous luster.
The Micas. The micas are common rock-making minerals. They may be divided into two classes: the light colored micas which are chiefly muscovite, and the dark colored micas consisting mostly of biotite. They are to be determined by their micaceous structure, eminent cleavage and the elasticity of their leaves. Muscovite is found in granites and syenites and other igneous rocks. It is especially common in the metamorphic rocks, particularly the gneisses and schits. Biotite is found in many igneous rocks such as the granites, syenites and felsites. It occurs also in the gneisses and schists.
The Pyroxenes. The pyroxenes form an important series of rock-making minerals which, although the different members vary considerably in composition, are closely related crystallographically. The important types are hypersthene, (Mg, Fe)SiO3, diopside, CaMg(SiO3)2, common pyroxene, Ca(Mg,Fe)(SiO3)2, augite, Ca(Mg,Fe)(Al,Fe)2SiO6 as well, and aeguirite, NaFe(SiO3)2. The pyroxenes are characteristically found in igneous rocks, particularly those that contain large amounts of lime, iron and magnesia, such as basalt, gabbro, periodite, etc. Diopside and common pyroxene are at times found in metamorphic limestones. The pyroxenes vary in color from white trough green to black. They occur usually in small-grains or in short prisms. It they show distinct crystal outlines, they can be told by the square cross section of their prisms. They have a rather poor cleavage.
The Amplibole. The ampliboles or horneblendes are calcium, magnesium, iron metasilicates which closely resemble the pyroxenes in their chemical composition. The most important menbers of the group are tremolite, CaMg3(SiO3)4, actinolite, Ca(Mg, Fe)3(SiO3)4, common horblende, Ca(Mg,Fe)3(SiO3)4, with a molecule containing aluminum and ferric iron besides, and arfvedsonite, which contain chiefly soda, lime and iron protoxide. The amphiboles are particularly characteristic of the metamorphic rocks, but are found in the igneous rocks as well. Tremolite is most commonly found in crystalline metamorphosed limestones, actinolite in schists, hornblende in granites, syenites and diorites, and also in gneisses and horblende achists. The amphiboles commonl occur in bladed prismatic crystals with a good prismatic cleavage. The cleavage angle is broad, having a value of about 125°. The vary in color from white through green to black, but are most commonly green.
Chysolite, or Olivine. Chrysolite, or olivine, as it is more commonly termed when spoken of as a rock constituent, is an orthosilicate of magnesium and ferrous iron (Mg, Fe)3(SiO3)4. It is a characteristic constituent of the ferromagnesian igneous rocks such as gabbros , periodites and basalts. It is almost the only mineral present in the igneous rocks known as dunite. It is usually green in color, with a vitreous luster and granular structure.
Kaolin. Kaolin is a silicate of aluminum, H4Al2SiO9, which is always secondary in its origin. It is formed by the weathering of some aluminum silicate, usually a feldspar. It may occurs in quite pure masses where feldspathic rocks have been entirely altered, but is most commonly found, however, in am impure state in clay, and in the rocks formed from claylike materials such as shales, slates, etc. When pure it is often friable or mealy in structure, alhough at times it is compact. It varies in color from white to yellow, red, etc., depending upon the amount and character of the foreign material mixed with it
Clorites. The chlorites are a group of green-colored micaceous minerals of which clinochlore is the most common member. It composition they are hydrous silicates of aluminum and magnesium. They are always secondary in their origin. They are frequently formed by the alteration of he ferromagnesian minerals occurring in igneous rocks. The green color of such rocks is usually due to the presence of chlorite. They are also common in the chlorite-shists, in green color, micaceous structure, perfect cleavage, and by the fact that their leaves are not elastic.
Serpentine. Serpentine, H4Mg3SiO9, is also a secondary mineral formed by the alteration of some original ferromagnesian mineral, such as pyroxene, amplybole, and especially olivine. It occurs, therefore, in altared igneous rocks and metamorphic rocks. It may occur in disseminated particles or in rock masses, of which it is the chief mineral. It is usually of some shade of green in color and has an oily or waxy luster. It is usually massive in structure, but may become coarsely fibrous in the variety known as chrysotile.
Talc. Talc, H2Mg3(SiO3)4, is similar in its oigin and occurrence to serpentine. It found at times in altered igneous rocks, but is more characteristic of metamorphic rocks where it may occur in large beds as soapstone. It is characterized by its extreme softness (1), greasy feel and also frequently by its foliated structure.
Calcite. Calcite, CaCO2, is a common and widely distributed rock – making mineral found chiefly in the sedimentary and metamorphic rocks. Such rocks as the limestones, marbles and chalks are composed almost entirely of the mineral. It is to be told by is softness (3), its rhombohedral cleavage and its ready effervescence in cold acids.
Dolomite. Dolomite, CaMg(CO3)2, is found in the same way as calcite but less commonly. The two minerals are usually associated whit each other and form dolomite marbles and dolomitic limestones. Its physical properties are practically the same as those of calcite. It will only effervesce, however, in hot acids.
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