Nosite—nosean; a complex sodium-aluminum silicate and sulphate, like haüynite, but with little calcium; common in phonolites.
Oligoclase—a plagioclase feldspar; essentially an aluminum-calcium-sodium silicate which may be regarded as a mixture of albite and a small amount of anorthite; triclinic; luster vitreous, pearly, or waxy; whitish grading into greenish and reddish; H. 6–7; common in crystalline rocks.
Orthoclase—a potash feldspar; essentially a potassium-aluminum silicate; varying by the replacement of the potassium by sodium and less frequently by other substitutions; monoclinic; occurring in distinct crystals and also in cryptocrystalline forms; cleavage planes perfect with pearly luster on cleavage surface; white, gray, and flesh-red, occasionally varying to greenish white and bright green; H. 6–6.5; difficultly fusible; sanidine a glassy variety; felsite a cryptocrystalline form; a very common mineral, especially in the granites and gneisses.
Olivine—chrysolite (q.v.).
Omphacite—a variety of pyroxene of grass-green color and silky to fibrous luster; allied to diallage.
Opal—silica with a varying amount of water; differs from quartz in a lack of crystallization and in lower degree of hardness; amorphous, massive; sometimes reniform, stalactitic, or tuberous; also earthy; H. 5.5–6.5; luster vitreous, inclining to resinous; white, yellow, red, brown, green, gray, blue, generally pale; colors arise from admixtures; sometimes play of colors as in precious opal.
Ozocerite—a native paraffine, mineral wax.
Petroleum—naphtha; a native mineral oil; a hydrocarbon, commonly believed to arise from organic matter, both animal and vegetable, but held by some to be due to deep-seated chemical and thermal action.
Pictotite—a variety of spinel, containing chromium.
Pisolite—a concretionary variety of calcite.