Picrolite—a variety of serpentine.
Piedmontite—a manganese epidote.
Plagioclase—a general term embracing the triclinic feldspars whose two cleavages are oblique to each other; embracing albite, oligoclase, andesine, labradorite, and anorthite (q.v.).
Plumbago—graphite (q.v.).
Psilomelane—essentially a hydrous manganese oxide occurring in massive, botryoidal, reniform, and stalactitic forms; luster submetallic; iron-black, passing into dark steel-gray; H. 5–6; the common ore of manganese.
Pseudomorph—a false form, i.e., having the form of one mineral and the composition of another; usually arises from the replacement of a mineral, particle by particle, by a solution of another substance, leaving the original form unchanged.
Pyrite—iron pyrites, fool’s gold, iron sulphide; isometric; commonly in cubes; H. 6–6.5; luster metallic, splendent, or glistening; pale brass-yellow; occurs widely disseminated throughout a large class of rocks; usually harder and lighter in color than copper pyrites, and deeper in color than marcasite, which has the same composition.
Pyroxene—the type of a large and important group of rock-forming ferromagnesian minerals; varies in composition and embraces a large number of varieties; usually a magnesium-iron-calcium silicate; crystals usually thick and stout, but varying greatly; sometimes lamellar and fibrous; H. 5–6; luster vitreous inclining to resinous; green of various shades verging towards light colors, occasionally more often to browns and blacks; among the minerals belonging to the pyroxene group are augite, bronzite, diallage, diopside, enstatite, hypersthene, and others.
Quartz—crystallized silica; rhombohedral; crystals commonly six-sided prisms capped by six-sided pyramids; without cleavage; H. 7; scratches glass; usually transparent, glassy, colorless when pure, shaded by impurities to yellow, red, brown, green, blue, and black; varieties, amethyst, purple, or violet; false topaz, yellow, rose-quartz, smoky, milky, cat’s eye, opalescent; aventurine, spangled with scales of mica; chalcedony is a cryptocrystalline variety; carnelian, a red chalcedony; chrysoprase, an apple-green chalcedony; prase, a leek-green variety; agate, a variegated or banded chalcedony; moss-agate, a chalcedony containing moss-like or dendritic crystallizations of iron or manganese oxide; onyx, a chalcedony in layers; sardonyx, like onyx in structure, but includes layers of sard (carnelian); jasper, an opaque-colored quartz, usually red or brown; flint, an opaque impure chalcedony; chert, an ill-defined term applied to an impure flinty rock; hornstone, a translucent, brittle, flinty rock.
Rutile—titanium oxide; tetragonal, crystals commonly in prisms; H. 6–6.5; luster metallic, adamantine; reddish brown, passing to red; sometimes yellowish, bluish, violet, and black; occurs in crystalline rocks and is a common secondary product in the form of microlites.