Facts about quartz
Quartz has attracted attention from the earliest times;
water-clear crystals were known to the ancient Greeks as
krystallos—hence the name crystal, or more commonly rock
crystal, applied to this variety. The name quartz is an old German
word of uncertain origin first used by Georgius Agricola in
1530.
Quartz is the second most abundant mineral in the Earth's crust.
It occurs in nearly all acid igneous, metamorphic, and sedimentary
rocks. It is an essential mineral in such silica-rich felsic rocks
as granites, granodiorites, and rhyolites. It is highly resistant
to weathering and tends to concentrate in sandstones and other
detrital rocks. Secondary quartz serves as a cement in sedimentary
rocks of this kind, forming overgrowths on detrital grains.
Microcrystalline varieties of silica known as chert, flint, agate,
and jasper consist of a fine network of quartz. Metamorphism of
quartz-bearing igneous and sedimentary rocks typically increases
the amount of quartz and its grain size.
Quartz exists in two forms: (1) alpha-, or low, quartz, which is
stable up to 573° C (1,063° F), and (2) beta-, or high, quartz,
stable above 573° C. The two are closely related, with only small
movements of their constituent atoms during the alpha-beta
transition. The structure of beta-quartz is hexagonal, with either
a left- or right-handed symmetry group equally populated in
crystals. The structure of alpha-quartz is trigonal, again with
either a right- or left-handed symmetry group. At the transition
temperature the tetrahedral framework of beta-quartz twists,
resulting in the symmetry of alpha-quartz; atoms move from special
space group positions to more general positions. At temperatures
above 867° C (1,593° F), beta-quartz changes into tridymite, but
the transformation is very slow because bond breaking takes place
to form a more open structure. At very high pressures alpha-quartz
transforms into coesite and, at still higher pressures, stishovite.
Such phases have been observed in impact craters.
Quartz is piezoelectric: a crystal develops positive and
negative charges on alternate prism edges when it is subjected to
pressure or tension. The charges are proportional to the change in
pressure. Because of its piezoelectric property, a quartz plate can
be used as a pressure gauge, as in depth-sounding apparatus.
Just as compression and tension produce opposite charges, the
converse effect is that alternating opposite charges will cause
alternating expansion and contraction. A section cut from a quartz
crystal with definite orientation and dimensions has a natural
frequency of this expansion and contraction (i.e., vibration) that
is very high, measured in millions of vibrations per second.
Quartz. (2010). In Encyclopædia Britannica. Retrieved March
20, 2010, from Encyclopædia Britannica Online:
http://search.eb.com.ezproxy.ub.gu.se/eb/article-9062182
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Fakta om kvarts
Kvarts (ty. Quarz, ett ord av omdiskuterat ursprung, möjligen
till ty. dial. Querk 'dvärg'; jfr kobolt), Det förekommer i två
modifikationer: lågkvarts och högkvarts. Lågkvarts är stabil vid
temperaturer under 573 °C. Stabilitetsområdet för högkvarts är
mellan 573 och 867 °C, varför detta mineral normalt inte förekommer
inom den övre jordskorpan. Lågkvarts har starka piezo- och
pyroelektriska egenskaper samt är optiskt aktiv (enantiomeri).
Kvarts är polymorf med tridymit, cristobalit, coesit och stishovit.
Jfr kiseldioxid.
Kvarts är den kontinentala jordskorpans näst vanligaste mineral
(11 volymprocent). Det förekommer såväl i sura och intermediära
magmatiska bergarter som i impregnationer och sprickfyllnader i
jordskorpan. Då kvarts är stabil under låg- till högmetamorfa
förhållanden förekommer mineralet såväl i fylliter, skiffrar och
gnejser som i granuliter. Som vittringsrest finner man kvarts
anrikad i lösa sediment, t.ex. kvartssand, och i sedimentära
bergarter, t.ex. sandsten.
Inom Sverige bryts kvarts i Näshult sydöst om Tranemo (Västra
Götalands län) och på några ställen i Dalsland.
2010-03-20 Nationalencyklopedin
http://www.ne.se.ezproxy.ub.gu.se/lang/kvart
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