Oxides, Oxides of Silicon and Quartz Composition
The oxides are subdivided into three sections:
Oxides of Silicon
Quartz
Composition. Silicon dioxide, SiO2 = Oxygen 53.3, silicon 46.7. Often with various impurities.
Crystallization. Hexagonal – rhombohedral; tramexohedral. Crystals commonly prismatic, with prism faces horizontally striated. Terminate usually by a combination of a positive and negative rhombohedron, which often are so equally developed as to give the effect of a hexagonal pyramid (Fig. 248). Sometimes one rhombohedron predominates or occurs alone (Fig. 249). At times the prism faces are wanting, and the combination of the two rhombohedrons gives what appears to be a doubly terminated hexagonal pyramid (known as a quartzoid). Crystals at times very much distorted, when the recognition of the prism faces by their horizontal striations faces are to be occasionally observed as small truncations between a prism face and that of an adjoining rhombohedron either to the right or left, forming what are known as right – or left – handed crystals. Crystals are often elongated in tapering and sharply pointed forms, due to an oscillatory combination between the faces of the different rhombohedrons and those of the prism (A, P1. VI). Sometimes twisted and bent.
Crystals frequently twinned. The twins at times are so intimately intergrown that they can only be determined by the irregular position of the trapezohedral faces, bye etching the crystal or bye the pyroelectric phenomena that they show.
Structure. Commonly in crystals. From large crystals usually attached at one end, to finely crystalline coating, forming “drusy” surfaces. Also common in massive forms great variety. From coarse – to fine – grained crystalline to flintlike or cryptocrystalline varuetues. Sometimes in concretionary forms, mammillary, etc. as sand.
Physical Properties. H. = 7. G. = 2.65 – 2.66. Vitreous luster, sometimes greasy, splendent to nearly dull. Color widely various. Usually colorless or white, but frequently colored by various impurities, yellow, red, pink, amethyst, green, blue, brown, black. Transparent to opaque. Conchoidal fracture.
Test. Infusible. Insoluble. Yields a clear glass when the finely powdered mineral is fused with an equal volume of sodium carbonate. Usually told by its glassy luster, conchoidal fracture, hardness (7) and crystal form.
Varieties. A great many different forms of quartz exist to which varietals names have been given. The more important varieties with a brief description of each follow.
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