Fig. 75.—Crystal forms cf common minerals: a, galena (Isometric); b, sphalerite (Isometric); c, beryl (Hexagonal); d, hematite (Hexagonal); e, magnetite (Isometric); f, barite (Orthorhombic); g, apatite (Hexagonal); h. sulphur (Monoclinic); i, gypsum (Monoclinic); j, chalcopyrite (Tetragonal); k, fluorite (Isometric); l, zircon (Tetragonal); m, tourmaline (Trigonal); n, corundum (Hexagonal).
Beryl. A silicate of aluminum and the rare chemical element beryllium. Hexagonal crystals usually of very simple six-sided prismatic habit (see [Figure 75c]). Color white, green, blue, or yellow. Specific gravity, 2.8. Cleavage practically absent. It is a very exceptionally hard mineral, being 8 in the scale. Very large crystals have been found, as, for example, in New Hampshire, where single crystals several feet long weigh a ton or more. Beryl is also of special interest because two of its varieties—emerald (green) and aquamarine (blue)—are well-known gem stones, the emerald being one of the most highly prized gems. The colors are due to slight impurities. Beryl most commonly occurs in dikes of coarse granite called pegmatite, but also in certain metamorphic and sedimentary rocks.
Calcite. Commonly called “calc spar.” A carbonate of lime. Hexagonal crystals in a great variety of forms, but all with crystal faces arranged in sixes around the principal or vertical axis forming rhombohedrons, prisms, or double-pointed pyramids. The principal axis of symmetry is sixfold by a combination of rotation and reflection. Very perfect cleavages in three directions yielding fragments whose faces make angles of 75 and 105 degrees. Color, white when pure, but variously colored when impure. Hardness, 3 (very easily scratched by a knife); specific gravity, 2.7. Calcite is a very common mineral, especially in limestone (including chalk) and marble which are usually largely made up of it. Also commonly found in veins, and as spring and cave deposits (stalactites). A porous, stringy variety, called travertine, is deposited by certain hot springs, as at Mammoth Hot Springs in Yellowstone Park. A very transparent crystalline variety is called Iceland spar. Calcite is a very useful mineral. Limestone and marble are widely used as a building stone, and for decorative purposes, statuary, etc. Limestone is burned for quicklime, used as a flux in smelting certain ores, in glass making, etc.
Cassiterite. The one great ore of tin whose composition is oxide of tin. Tetragonal crystallization ([Figure 73c]). Hardness greater than steel, being over 6 in the scale. Specific gravity 7, which is notably high. Color, brown to nearly black. Cleavage, practically absent. Fairly widespread in small amounts, and in commercial quantities in only a few localities, usually in veins in granite or metamorphic rocks near granite, as at Cornwall, England, also in the form of rounded masses in gravel deposition as in the Malay region.
Chalcocite. Crystallizes in the orthorhombic system, usually in tabular form, but crystals not common. A black sulphide of copper with metallic luster. Hardness, nearly 3; specific gravity, nearly 6. No cleavage. Chalcocite occurs in vein deposits as one of the important copper ores, especially at Butte, Montana.
Chalcopyrite. Known as “copper pyrites,” ([Figure 75j]). A deep brass-yellow sulphide of iron and copper. Seldom crystallized in tetragonal forms. Hardness, 3.5; specific gravity, over 4. No cleavage. Metallic luster. Widely distributed in vein deposits associated with other metal-bearing minerals. A very important ore of copper, especially at Rio Tinto, Spain.
Chlorite. A soft, green mineral, usually in small tabular crystals, in general appearance much like mica (see [below]), but unlike mica, the almost perfect cleavage leaves are not elastic, though they are flexible. Composition, a silicate of aluminum and magnesia. Always of secondary origin as a result of chemical alteration of certain other minerals, such as biotite-mica, pyroxene or amphibole.
Cinnabar. A vermilion-red sulphide of mercury. An extra soft metallic mineral, only 2.5 in the scale. Specific gravity over 8, which is notably high. Completely vaporizes on being heated. Small trigonal crystals rare. Cinnabar is the one great ore of mercury, occurring in veins, especially in California and Spain.
Copper. Copper as such (so-called “native copper”) is widely distributed in veins, usually in small amounts with other copper minerals, but in the great mines of northern Michigan it occurs in immense quantities as the only important ore. It is readily recognized by its color, softness (less than 3), and notable weight (specific gravity, nearly 9). Isometric crystals uncommon.
Corundum. An oxide of aluminum of hexagonal crystallization, usually in six-sided prisms, capped by very steep pyramidal faces (see [Figure 75n]). It is next to the hardest of all known minerals (9 in the scale), the diamond only exceeding it. Specific gravity about 4. Three good cleavages making angles of nearly 90 degrees with each other. The color of corundum is usually brown, but it varies greatly. Two of the most highly prized of all precious stones—ruby (red) and sapphire (blue)—are nearly transparent varieties of corundum, colored by certain impurities. Oriental topaz (yellow), oriental emerald (green), and oriental amethyst (purple) are also clear varieties of corundum. It occurs in various igneous and metamorphic rocks, and in some stream gravels. The finest rubies, associated with some sapphires, occur in gravels in Burma, Siam, and Ceylon. Emery is a fine-grained mixture of corundum and other minerals, especially magnetite.