Quartz

Quartz occurs in two distinct types, the macrocrystalline and the cryptocrystalline varieties, the latter often being classed as chalcedony. Chalcedony consists of quartz crystallites, commonly fibrous in form and with submicroscopic pores that cause it to have a somewhat lower density than macrocrystalline quartz. Many names are currently applied to different varieties of quartz; some of the more common names and characteristics of the material to which they are applied are as follows:

• Rock Crystal: transparent colorless material (at some localities, this variety has been given such names as Herkimer diamonds).
• Milky: milk-white and almost opaque, luster typically somewhat greasy; occurs in large masses in veins and pegmatites.
• Amethyst: transparent purple material, much used as an ornamental and semiprecious stone; most occurs lining cavities in volcanic rocks.
• Rose: rose-red or pink, rarely as crystals.
• Yellow (citrine): yellow and transparent; cut stones are sometimes sold as topaz. Amethyst on heating often turns yellow and much citrine is actually heat-treated amethyst.
• Smoky: transparent and semitransparent brown, gray to nearly black varieties; sometimes called cairn gorm because material from Cairngorm in Scotland has been widely used as an ornamental stone. Radiation from radioactive material often causes a smoky appearance in colorless quartz, and this process may be responsible for much of the smoky quartz in nature.

• Carnelian and Sard: red and reddish brown chalcedony.
• Heliotrope and Bloodstone: green chalcedony with red spots.
• Agate: banded forms of chalcedony, the banding being due to interrupted deposition in cavities, the bands typically being parallel to the walls of the cavity. Some natural agate shows attractive color banding, but much of the colored agate used for ornaments has been artificially dyed, the slightly porous nature of the chalcedony allowing it to absorb material from solution. Onyx is the name generally given to plane-banded, alternately light and dark agate; it is used widely for carved cameos.
• Moss Agate: white or cream-colored chalcedony enclosing brown or black dendritic mosslike aggregates of manganese oxides.

  Jasper: opaque chalcedony, generally red, yellow, or brown, the color being due to included colloidal particles of iron oxides.

• Chert and Flint: massive opaque chalcedony, typically white, pale yellow, brownish, gray, or black, and occurring as nodules or extensive beds in sedimentary rocks. The wide occurrence, the hardness and toughness, the conchoidal fracture, and the sharp cutting edge led to the widespread use of flint and chert by primitive peoples for tools and weapons.
• Petrified Wood: generally consists of reddish or brown chalcedony.
• Tiger’s eye: quartz pseudomorphous after asbestos and retaining the fibrous structure of that mineral. Most of it is yellow and brown or yellow and blue. It is extensively used as an ornamental stone, especially in jewelry.

Macrocrystalline quartz is generally rather easily recognized by its crystal form, hardness, and lack of cleavage; chalcedonic varieties are identified by their hardness and typical dense structure. Quartz is piezoelectric and pyroelectric. It is insoluble in all acids except hydrofluoric acid.

Quartz is stable over practically the whole range of geological conditions, and because Si02 is the most abundant oxide in the earth’s crust, it is a very common mineral. It is present in silica-rich igneous rocks, both volcanic and plutonic, and it makes up a large part of hydrothermal veins and granite pegmatites. Since it is hard and extremely resistant to chemical weathering, it is the most abundant detrital mineral and the basic constituent of most sandstones. It is an important constituent in most metamorphic rocks. The different varieties of chalcedony occur in sedimentary rocks and in veins and cavities. The uses of quartz are many and diverse. Enormous tonnages are used in the construction industry: quartzite and sandstone are used as building stone, as aggregate in concrete, and as sand in mortar and cement. Large amounts are used as a flux in metallurgy; in the manufacture of glass, ceramics, and refractories; as an abrasive; and as a filler. Many of the colored varieties are cut and polished as ornamental and semiprecious stones. Fused silica is a useful material because of its chemical inertness and its low coefficient of expansion. Dishes and crucibles of fused silica can be heated and then chilled rapidly without danger of breakage. One of the most interesting applications of quartz crystals makes use of their piezoelectric properties to measure pressures and to control the frequency of electrical impulses; although the total amount used annually for these purposes is comparatively small, the supply of satisfactory natural crystals (mainly from Brazil) is not always adequate, and so laboratory methods of growing suitable crystals have been developed. 

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