Silicates: Quartz

Composition: SiO₂; the coarsely crystalline variety is called quartz, the cryptocrystalline variety is called chalcedony

Crystal system: Trigonal; crystals commonly prismatic with variably developed terminations, also massive to granular or as discrete grains in sands and gravels

5.22.1
quartz crystal

Properties:

• Vitreous lustre
• Variable color; colorless, white, purple, pink, yellow, brown to nearly black
• White streak
• Knife does not scratch (Mohs Hardness 7)
• Average density
• No cleavage
• Smooth (conchoidal) fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Quartz is an extremely common mineral. It occurs as a major mineral phase in silica-rich igneous rocks, many sandstones and metamorphic rocks, in sand and gravel deposits and with precious metals, sulfides and carbonates in hydrothermal vein deposits.

5.22.2 vein quartz in pyritic wallrock Mt Charlotte mine WA Australia	5.22.3 vein quartz in pyritic wallrock Mt Charlotte mine WA Australia

5.22.4 quartz crystal	5.22.5 massive rose quartz Custer South Dakota USA

5.22.6 amethyst crystals	5.22.7 amethyst crystals

Quartz is an important industrial mineral used in many industries, particularly construction, glass manufacturing and casting. Gem quality quartz occurs in a variety of colors including purple (amethyst), pink (rose), yellow (citrine) and brown to black (smoky) as a result of differing contents of trace elements and the effects of natural radiation.