Quartz occurs in two basic forms:
Quartz Varieties
Occurrence of Quartz
Quartz as a Rock-Forming Mineral
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Категория: ГеографияГеография


1. Quartz


Colour: Colorless, purple, rose, red, black, yellow, brown, green, blue, orange,
Lustre: Vitreous
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.


4. 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.

6. 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.

7. 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 righthanded forms individually). It is convenient and common practice to
designate them with Latin and Greek letter symbols instead of MillerBravais 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
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


11. 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.

12. 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 silicarich 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 rockforming 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.

13. Спасибо за внимание

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