Pyrolusite.—Black oxide of manganese, MnO2, though generally impure from admixture with other manganese oxides. Usually in intricate aggregates which may be columnar, fibrous, mammillary, earthy, etc. Opaque, with color and streak both black. Soft and easily soils the fingers. With hydrochloric acid gives off the choking fumes of chlorine. Hardness 2-2.5. Specific gravity 4.8.
II. The Minerals important as Rock Makers
These minerals are in most cases complex silicates of one or more of a certain number of metals such as aluminium, calcium, magnesium, iron, sodium, potassium, or hydroxyl (OH). For their identification an examination of the physical properties is usually sufficient, whereas of the typical ore minerals already considered, additional chemical tests may be necessary.
Feldspars.—A group of similar alumino-silicates of potassium, sodium, and calcium. The most important of all rock-making minerals. Although with wide variation in chemical composition, the feldspars are yet broadly divided into two classes; the one striated, and the other an unstriated potash or orthoclase variety. The pocket lens is usually necessary in order to make out the striations upon the crystal or cleavage surfaces. When formed in veins, feldspar appears in crystals ([Fig. 487], 5-6), but as a rock constituent the mutual interference of crystals prevents the development of bounding faces. Two cleavage directions, nearly or quite perpendicular to each other, are notably different in their perfection. Hard enough to scratch glass, but easily scratched by sand. Color pink (usually orthoclase or microline), white (often albite) to gray. Sometimes with beautiful “pigeon’s throat” effect of iridescence (labradorite). Low specific gravity. Hardness 6. Specific gravity 2.5-2.8.
Fig. 487.—Forms of Crystals: 1-2, gypsum; 3-4, cerussite; 5-6, feldspar; 7, quartz; 8, pyroxene (cross section); 9, hornblende (cross section); 10, garnet; 11, nephelite; 12-14, staurolite; 15-16, tourmaline (cross sections); 17, olivine.
Quartz.—Oxide of silicon or silica, SiO2. Both an important vein mineral associated with the ores and a rock maker. In the former case particularly, often in crystals of notably simple forms ([Fig. 487], 7). Few minerals which are not gems are so hard. Remarkable freedom from cleavage so that the mineral breaks much like window glass—conchoidal fracture. Wide range in both transparency and color. Transparent and colorless crystalline variety (rock crystal), brown translucent (smoky quartz), turbid white (milky quartz), and various colored varieties (carnelian, jasper, jet, etc.). Insoluble in acids and infusible. Hardness 7. Specific gravity 2.6.
Micas.—Like the feldspars a group of complex silicates, but here chiefly of potassium, magnesium, iron, and hydroxyl. Abundant as rock makers, the micas are all characterized by the thinnest and toughest of elastic cleavage plates, such as are generally known as isinglass. When a needle is driven sharply through a thin scale of mica, a six-rayed puncture star forms about the needle point. The darker common variety of mica is rich in iron and magnesium and is called biotite, and the lighter colored alkaline variety, muscovite. Hardness 2.5-3.1. Specific gravity 2.7-3.1.