Quartz презентация

Formula:SiO2
Colour: Colorless, purple, rose, red, black, yellow, brown, green, blue, orange, etc.
Lustre: Vitreous
Hardness:7
Specific Gravity:2.65 - 2.66
Crystal System: Trigonal
Name: Quartz has been known and appreciated since pre-historic times. The most ancient name known is recorded by Theophrastus in about 300-325 BCE, κρύσταλλος or kristallos. The varietal names, rock crystal and bergcrystal, preserve the ancient usage. The root words κρύοσ signifying ice cold and στέλλειυ to contract (or solidify) suggest the ancient belief that kristallos was permanently solidified ice.

Quartz occurs in two basic forms:
1. The more common macrocrystalline quartz is made of visible crystals or grains. Examples are rock crystals, the grains in sandstone, but also massive quartz that is made of large crystallites without any crystal faces, like vein quartz.

2. Cryptocrystalline quartz or microcrystalline quartz is made of dense and compact aggregates of microscopic quartz crystals and crystallites. Examples are agate and chert. The different types of cryptocrystalline quartz are colloquially subsumed under the term chalcedony, although that term has a more strict definition in scientific literature. It is worth mentioning that most chalcedony contains small amounts of another SiO2 polymorph, moganite, so it is not always pure quartz.

Quartz Varieties
Quartz crystals or aggregates that share certain peculiar physical properties have been classified as quartz varieties with specific "trivial names".
The best known examples are the colored varieties of quartz, like amethyst or smoky quartz, but there are also trivial names for specific crystal shapes, aggregates and textures, like scepter quartz, gwindel or quartzine. Because there are no canonical rules on naming or defining quartz varieties like they are for minerals, the definitions of some quartz varieties are precise and generally accepted, while the definitions of others vary considerably between different authors, or are rather fuzzy.

Morphology
Quartz is found as individual crystals and as crystal aggregates. Well crystallized quartz crystals are typically six-sided prisms with steep pyramidal terminations. They can be stubby ("short prismatic") or elongated and even needle-like. In most environments quartz crystals are attached to the host rock and only have one tip, but double-terminated crystals are also found.
As a rock-forming mineral quartz commonly occurs as sub-millimeter to centimeter-sized anhedral grains, well-formed crystals are uncommon. Secondary vein-fillings of quartz are typically massive.

Quartz belongs to the trigonal-trapezohedral crystal class 32. Of the seven basic crystallographic forms of this crystal class the hexagonal prism and trigonal rhombohedra are very common and determine the overall shape of the crystals. The trigonal bipyramids and trigonal trapezohedra are frequently found, but typically only as relatively small faces. The trigonal prisms, the basal pinacoid and in particular ditrigonal prisms are very rare (Frondel, 1962).
Quartz crystals show about 80 different crystallographic forms in nature (Frondel, 1962; Rykart, 1995; Frondel lists 112, but counts left- and right-handed forms individually). It is convenient and common practice to designate them with Latin and Greek letter symbols instead of Miller-Bravais indices. The following figure illustrates the relation of the common forms (sorted by abundance) to the faces found on quartz crystals. The most common combination of crystallographic forms in quartz crystals is r+m+z.

The handedness of quartz crystals can be determined easily from the positions of x faces, which are at the lower left or lower right corner of the r face (orange faces in Fig.5). With some difficulty the handedness can be determined from the position of the s faces (blue faces in Fig.5), which lie between the r and z faces: the s face often shows a fine striation that runs parallel to the edge of the r-face.
The bottom row shows a top view of the crystals. It does not only show their trigonal symmetry but also the chirality of the position of the x faces.

Occurrence of Quartz
Quartz is one of the crystalline forms of silica, the essential building material for all silicates, and quartz can only form where silica is present in excess of what is consumed in the formation of other silicate minerals.
Quartz may also be consumed during the formation of new silicate minerals, in particular at higher temperatures and pressures, and certain geological environments are "incompatible" with free silica and hence quartz.

Quartz as a Rock-Forming Mineral
Silica has been enriched in the continental Earth's crust to about 60% (Rudnick and Gao, 2003) by processes like magmatic differentiation and the formation of silica-rich igneous rocks (mainly driven by plate tectonics) and the accumulation of the physically and chemically stable quartz in sediments and sedimentary rocks. The oceanic crust's silica content of about 50% (White and Klein, 2014) in its igneous rocks is too low for quartz to form in them.
The largest amount of quartz is found as a rock-forming mineral in silica-rich igneous rocks, namely granite-like plutonic rocks, and in the metamorphic rocks that are derived from them. Under conditions at or near the surface, quartz is generally more stable than most other rock-forming minerals and its accumulation in sediments leads to rocks that are highly enriched in quartz, like sandstones. Quartz is also a major constituent of sedimentary rocks whose high quartz content is not immediately obvious, like slates, as well as in the metamorphic rocks derived from such quartz-bearing precursor rocks.