Accessory Rock-making Minerals

In addition to more important and common rock-making minerals that have been descried in the preceding pages, there is a group of minerals which are characteristically found as rock constituents but in a minor way. They occur usually only a small and scattered crystals in the rock and seldom become one of its prime constituents. These minerals are known as accessory rock-making minerals. The occurrences of the more important of them are briefly described below.

In addition to more important and common rock-making minerals that have been descried in the preceding pages, there is a group of minerals which are characteristically found as rock constituents but in a minor way. They occur usually only a small and scattered crystals in the rock and seldom become one of its prime constituents. These minerals are known as accessory rock-making minerals. The occurrences of the more important of them are briefly described below.

Garnet. Garnet is a common accessory mineral, being particularly characteristic of the metamorphic rock. It is found frequently in mica- schists, horblende-schistes, gneisses and metamorphosed limestones. More rarely it is found in igneous rocks. It occurs in small irregular grains or frequently in fair-sized definitely shaped crystals . it is usually red or brown in color. For the different varieties of garnet and their distinguishing features, see p. 259.

Epidote. Epidote is formed by the alteration of silicates containing lime, iron and aluminum. It also characteristic of metamorphosed limestones. It may be associated with chlorite, calcite, etc. It usually found in bladed crystalline masses and has a characteristic yellow-green color, is hard and has one good cleavage.

Staurolite. Staurolite is found in metamorphic rocks, such as the mica-schists and slates. Sometimes it is a constituent of gneiss. It is associated with mica, quartz, garnet, cyanite, etc. It is  characterized by a brown color, hardness (7) and prismatic orthorhombic crystals which may show cross-shaped twins

Cyanite. Cyanite, Al2SiO5, is a rather rare accessory mineral which is found in gneisses and mica-schsts. It i to be distnguished by its bladed structure, one goog cleavage, blue color and by the fat that it is distinctly harder in the direction parallel to the length of the crystals than in the direction at right angles to this.

Zircon. Zircon, ZrSiO4, is a rather rare mineral which usually occurs in minute crystals scattered throughout a rock mass. It is found in granite, syenites, crystalline limestone, chlritic schists, etc. It is to distinguished by its usually brown color, hardness (7.5) and tetragonal crystallization.

Titanite. Titanite or sphene, CaTiSio5 is a comparatively rare mineral found as an accessory constituted in granite, syenites, gneiss, mica- and chlorite-schists and crystalline limestones. It occurs as microscopical crystals in many igneous rocks.

Magnetite. Magnetite, Fe3O4, is widespread in it occurrence as a rock constituent. It is found in all kinds of igneous rocks, usually in small disseminated grains. It is also characteristic of the crystalline schist and gneisses. Ordinarily it occurs in comparatively small amounts would be classed as an accessory mineral bu t at times it it becomes a prime constituent of the rock and may be segregated into almost pure bodies of the mineral. It is characterized by its metallic luster, black color and streak and its strong magnetic properties.

Ilmenite. Ilmenite or titanic iron, FeTiO3, is a common accessory mineral occurring in the same way as magnetite and frequently found associated with it. It is most commonly found in the gabbros and related rocks. It is difficult to tell it from magnetite by simple by simple inspection.

Heamatite. Hematite Fe2O3, is found as an accessory mineral in the feldspathic igneous rocks such as granite. It occurs also in the crystalline schists. It is common in the sedimentary and metamorphic rocks and times forms large bodies of almost pure mineral. It is the red pigments in many rocks and soils and forms the cementing material in many sandstones. It is to be recognized by its red streak.

Pyrite, FeS2, is found in small disseminated crystals in all classes of rocks. It is characterized by its pale brass color, metallic luster, hardness (6), black streak and frenquently also by its isometric crystals forms.

Apatite. Apatite, Ca4(CaF)(PO4)3, is found in crystals of considerable size in metamorphose limestones. It is also common in microscopic ones. In addition to the minerals listed abode, the following, more rare in their occurrence, are at times found as accessory rock constituents: rutile, iolite, scapolite, andalucite and sillimanite.

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