QUARTZ
Quartz (silicon dioxide/silica - SiO2) is
the most common mineral in the Earth's crust. It is composed of the two
most abundant elements in the crust - oxygen and silicon. It has a
glassy, nonmetallic luster, is commonly clearish to whitish to grayish in
color, has a white streak, is quite hard (H≡7), forms hexagonal crystals,
has no cleavage, and has conchoidal fracture. Quartz can be any
color: clear, white, gray, black, brown, pink, red, purple, blue, green,
orange, etc.
Quartz
(2.2 cm across) - clear quartz crystal ("rock crystal").
Quartz
- nice radiating cluster of clear quartz crystals from the Collier Creek Mine,
Montgomery County, Arkansas, USA.
Quartz
(above & below) - doubly terminated clear quartz (“Herkimer Diamond
Quartz”) from dolostone vugs.
Stratigraphy: Little Falls Formation, Upper Cambrian.
Locality:
near Herkimer, New York State, USA.
Above:
2.6 cm across.
Below:
stacked “Herkimer Diamonds”.
Capped quartz from St. Dennis, Cornwall, England.
(Wayne State University collection, Detroit, Michigan,
USA)
Quartz with phantom from Minas Gerais State, Brazil.
(Wayne State University collection, Detroit, Michigan,
USA)
Quartz
showing Japan-law twinning from the Otome Mine near Kohfu, Yamanashi
Prefecture, Japan. Twinning refers to the intergrowth of two or
more crystals of the same mineral. A Japan-law twin is one of
several types of contact twins, characterized by having an obvious contact
plane separating the crystals. (HMNH # 28144, Harvard Mineralogical
Museum, Harvard Museum of Natural History, Harvard University, Cambridge,
Massachusetts, USA)
Quartz geode (above & below; above: 9.3 cm across at its widest; below:
field of view ~3.8 cm across) - geodes form by precipitation of crystals in
cavities by fluids moving through rocks.
Concentrically-layered microcrystalline quartz
(“agate”) lines the walls of the geode, and is capped by clearish
macrocrystalline quartz.
Quartz
after fluorite from the Idarado Mine, Telluride Mining District, San Miguel
County, Colorado, USA. The cubic shapes
are leftover from the original fluorite crystals - quartz has replaced them.
(CSM # 51991, Colorado School of Mines Geology Museum,
Golden, Colorado, USA)
Most color varieties of quartz result from silica
mixed with impurities. Sometimes, coloration is provided by radiation
scorching or by the presence of abundant fluid inclusions.
Milky quartz (4.3 cm across) - the white coloration is attributed to the presence
of abundant microscopic fluid inclusions.
Milky quartz from the Ohio Mine near Ouray, Colorado, USA. (Cranbrook
Institute of Science specimen, Bloomfield Hills, Michigan, USA)
Rose Quartz (3.6 cm across) - the pink coloration of rose quartz has been
attributed to the presence of Ti+4 impurity or the presence of
dumortierite-like fibers. Some rose quartz coloration has been attributed
to radiation bombardment of quartz having Al and P impurities.
Amethyst
(purple quartz) - the coloring agent for amethyst is not agreed upon.
Some say its due to Fe+4, some say Fe+3, some say Mn.
The cluster of amethyst crystals shown above is from Russia. (CM
public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania,
USA)
Amethyst
(purple quartz) in large geode ("amethyst cathedral") from South
America.
Amethyst
(purple quartz) lining cavity from Rio Grande do Sul, Brazil (Colorado School
of Mines Geology Museum, Golden, Colorado, USA).
Amethyst
(purple quartz) from Vera Cruz, Mexico (Wayne State University specimen, Detroit,
Michigan, USA).
Smoky quartz in geode - smoky quartz is blackish or very dark gray or
brownish. The coloration is attributed to radiation scorching or the
presence of FeO or Mn impurities. Smoky quartz can also be simulated by
artificial irradiation of clear quartz.
Locality:
Sowerbutt Quarry, Prospect Park, New Jersey, USA.
Chrysoprase (3.0 cm across along the base) from Queensland, Australia.
Chrysoprase is an apple-green, microcrystalline "variety" of
quartz. The green color has been attributed to nickel oxide impurity.
Jasper
(4.2 cm across along the base) from Cave Creek, Arizona, USA. Jasper is
an intensely red, micro- to cryptocrystalline “variety” of quartz. The
red is from hematite (Fe2O3). Jasper is really not
a variety of quartz - it's a variety of chert, a sedimentary rock composed of
cryptocrystalline quartz.
There are numerous named "varieties" of
quartz. Many of the names are inconsistently applied by the general
public and by geologists.
Agate
(cut & polished slice, 7.4 cm across) - the term agate is usually
used to refer to irregularly- & concentrically-layered masses of
microcrystalline quartz. Individual layers consisting of
microcrystalline, fibrous quartz (either translucent or opaque) are said to be chalcedony.
The different layers vary in color due to the presence of various
impurities. Many agate masses are simply geodes that have completely
filled up with quartz. Common agate colors are clearish-whitish-grayish,
brownish-red, and yellowish-brown. Commercial agates that occur in greens
and blues and purples are almost always dyed (faked).
Agate
(11.0 cm across) - cut & polished nodule.
Agate-filled
geodes (above & below) from the Las Choyas Geode Deposit in northern Aldama
County, north-central Chihuahua State, northern Mexico. Geodes at this
locality occur in structurally-folded, rhyolitic volcanic tuffs (ash flow
tuffs) of Middle to Late Eocene age (~35-44 m.y.). The geodes were
originally cavities in the rhyolitic rock. These cavities (lithophysae)
formed before the rock completely lithified. The original ash flow
deposit had some subspherical structures known as spherulites, composed of
glassy to cryptocrystalline material (many felsic extrusive igneous rocks have
these). Expanding gases in the spherulites destroyed the material, resulting
in empty spaces. In the near-latest Eocene (~35 m.y.), regional rhyolite
dome intrusions resulted in hot groundwater percolating through the rocks,
leaching out silica and precipitating quartz in the lithophysae/cavities.
Specimens owned by Jeff Smith.
(Info. from Smith, 2010, The Las Choyas Geode Deposit,
Chihuahua, Mexico, Rocks & Minerals 85: 112-122 and Keller,
1977, Quartz geodes from near the Sierra Gallego area, Chihuahua, Mexico, Mineralogical
Record 10: 207-212.)
Agate
from Adrasman, Tajikistan. Specimen owned by Jerry Schaber.
Agate
from Ankara, Turkey. Specimen owned by Jerry Schaber.
Agate
from Dryhead, Montana, USA. Specimen owned by Jerry Schaber.
Agate
from Madagascar. Specimen owned by Jerry Schaber.
Agate
from Madagascar. Specimen owned by Jerry Schaber.
Agate
from Nowy Kosicol, Poland. Specimen owned by Jerry Schaber.
Agate
from Rio de Sol, Brazil. Specimen owned by Jerry Schaber.
Agate
from Wave Hill, Australia. Specimen owned by Jerry Schaber.
Agate
from Wave Hill, Australia. Specimen owned by Jerry Schaber.
Agate
from Wave Hill, Australia. Specimen owned by Jerry Schaber.
“Holley Blue Agate” (top specimen is 3.9 cm
across) - pale purplish chalcedony from Linn County, western Oregon, USA.
This attractive material is famous, but is never accompanied with useful
information, in terms of specific locality or geologic context. I don't
know what the host rock is, or the stratigraphy, or the age. I suspect
that this chalcedony fills vesicles in Cenozoic-aged lava. The locality
is not specified anywhere in the literature. The best locality
information I came up with is “Holley Mountain” (wherever that is) near the
towns of Holley or Sweet Home or Lebanon, Linn County, western Oregon, USA.
Specimens donated by James Cheshire.
Tiger-eye quartz (above & below; above: unpolished; below:
polished) - tiger-eye quartz consists of orangish-brown or reddish-colored,
compact masses of fibrous appearance that display chatoyancy (when tilted in
the light, a moving wavy silky sheen can be seen). It forms as a result
of quartz replacing asbestiform amphibole.
Star quartz from Germany. Each "star" is a radiating spray of twinned
quartz crystals. (Wayne State University collection, Detroit, Michigan, USA)
Sometimes, quartz coloration is due to the presence of
other minerals included within the crystal structure of quartz.
Quartz
cluster colored blue by the presence of papagoite (CaCuAlSi2O6(OH)3
- calcium copper hydroxy-aluminosilicate); from Messina, South Africa. (CM
public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania,
USA)
Quartz
(cut & polished slice) with inclusions of tourmaline (black lines &
sticks) and hematite (scattered red spots) from Minas Gerais, Brazil. (CMC
RM 1146, Cincinnati Museum Center's rock & mineral collection, Cincinnati,
Ohio, USA)
Rutilated quartz - quartz crystal with inclusions of long,
pale-colored needles of rutile (TiO2). (CM public display, Carnegie
Museum of Natural History, Pittsburgh, Pennsylvania, USA)
Quartz with inclusions of asbestos from Savois Department, France.
(Wayne State University collection, Detroit, Michigan,
USA)
Quartz
with hematite inclusions (red) on sphalerite (black) from a skarn deposit in
the Second Sovietskiy Mine at Dalnegorsk, Maritime Province, eastern Russia.
Photo gallery of quartz &
its varieties