Images
Formula: SiO2
Tectosilicate (framework silicate)
Varieties
Amethyst is a variety of quartz coloured by Fe3+ substituting for Si in tetrahedral co-ordination,
and then the action of natural irradiation producing Fe4+.
Blue quartz is a blue variety of quartz. Some examples of blue quartz are coloured by submicron-size inclusions
of ilmenite which produce scattering.
Other examples owe their colour to submicron size inclusions of rutile,
tourmaline or amphibole, and in rare
cases to incorporation of cobalt.
Citrine is a variety of quartz coloured yellow by submicroscopic distribution of colloidal ferric hydroxide
and oxides, as well as Fe3+ substituting for Si.
Faden quartz is a group of quartz crystals, usually tabular, with a white thread-like zone running through
the interior.
Ferruginous quartz is coloured red-brown by inclusions of hematite.
Green quartz owes its colour to inclusions of chlorite.
Milky quartz is a semi-transparent to opaque white-coloured variety of quartz.
Rose quartz is a variety of quartz which, when massive, is coloured by scattering of tiny
orientated rutile needles and/or the presence of Ti3+ in channels
and voids. Al3+ is usually also present. Transparent single crystals of rose quartz are coloured by
substitutional phosphorus.
Smoky quartz is a variety of quartz coloured dark brown to black due to the presence of Al3+ in
the tetrahedral site. If Fe3+ is present in greater concentration than Al3+, irradiation of clear
quartz produces smoky quartz at first, but further irradiation further ionises the iron
and causes charge transfer
between Fe4+ and a trapped hole on an oxygen atom of the Al3+ tetrahedron producing amethyst.
Properties of quartz
Crystal System: Trigonal
Specific gravity: 2.65 to 2.66 measured, 2.66 calculated
Hardness: 7
Streak: White
Colour: White or colourless, also grey, yellow, purple, pink, brown, black. Also may be coloured by blue, green
or red-brown by inclusions of other minerals.
Solubility of quartz
In water, hydrochloric, sulphuric and nitric acid: quartz is insoluble at atmospheric pressure and temperature, but
solubility increases rapidly with pressure and temperature.
In alkalis: SiO2 is attacked by alkaline substances (like potassium hydroxide, KOH). The speed of the reaction
depends on the texture and crystal size: crystalline quartz will dissolve only very slowly in hot watery alkaline
solutions, while amorphous SiO2, will be readily dissolved at room temperatures, according to the
equation:
SiO2 + 2KOH → K2SiO3 + H2O
In hydrofluoric acid: Hydrofluoric acid, HF, will decompose quartz to form first silicon fluoride SiF4, then
hydrofluorosilicic acid, H2SiF6:
SiO2 + 4HF → SiF4 + 2H2O
SiF4 + 2HF → H2SiF6
In natron or potash: All forms of silica dissolve in molten
natron or potash (K2CO3)
SiO2 + Na2CO3.10H2O → Na2SiO3 + CO2
+ 10H2O
SiO2 + K2CO3 → K2SiO3 + CO2
Weathering of silicate rocks: Carbonic acid
H2CO3 present, for example, in meteoric waters, releases silicic acid and forms carbonates.
Formation of orthosilicic acid: Quartz dissolves in water at sufficiently high pressure and temperature forming
orthosilicic acid,
H4SiO4:
SiO2 + 2H2O → H4SiO4
Orthosilicic acid is a very weak acid, weaker than carbonic acid. It dissociates according
to the reaction:
H4SiO4 + H2O ⇌ H3SiO4- +
H3O+
Environments: Quartz occurs in every type of mineral environment.
In the Bowen reaction series quartz is the last major mineral
to crystallise out.
It is the most common mineral found on the surface of the Earth; it occurs in plutonic igneous environments including
pegmatites and
carbonatites, in sedimentary environments,
in contact and
regional metamorphic environments, in hydrothermal
deposits and it is the principal constituent of hydrothermal veins.
Smoky quartz occurs in alpine fissures and veins; rose quartz occurs in
pegmatites.
Quartz is generally a primary, rock-forming mineral, but
it may also be of secondary origin.
It is an essential constituent of
quartzolite,
granite,
pegmatites,
rhyolite and
sandstone.
It is a common constituent of
diorite,
basalt,
phyllite,
gneiss and
eclogite.
Quartz also may be found in
syenite,
gabbro,
trachyte,
andesite,
clay,
limestone and
dolostone.
Quartz occurs in all metamorphic facies with the possible exception of the
sanidite facies, where the high temperature
paramorph tridymite
may occur instead.
Alteration
Quartz is ubiquitous and a component of many reactions which are detailed elsewhere, according to the
other components.
The only alterations considered here are transformations between the different
paramorphs of SiO2.
Alpha quartz is the low temperature, low pressure paramorph. At atmospheric
pressure it is stable up to
573oC, when it alters to beta quartz, the beta quartz alters to
tridymite at 870oC.
Alpha quartz, beta quartz and coesite can
co-exist at a point where the temperature is about 1,360oC and the pressure 34 kbar.
At atmospheric pressure and 1,470oC tridymite alters to
cristobalite, and cristobalite
melts at 1705oC.
Tridymite, cristobalite and
beta quartz can co-exist in
equilibrium at a point with temperature about 1,400 oC and pressure 30 kbar.
Coesite is a high pressure
paramorph of quartz.
With increasing pressure, at
800oC alpha quartz alters to coesite at
about 30 kbar
pressure, then
coesite alters to the ultra-high pressure
paramorph
stishovite at about 90 kbar.
beta quartz and coesite can co-exist in equilibrium
with the silica
melt at a point
where the temperature is about 2,410oC and the pressure 45 kbar.
With further increase in pressure and temerature, coesite
can continue to
exist
up to about 2,770oC and
110 kbar pressure, at which point coesite,
stishovite and the silica melt
are in equilibrium
(QP).
The Tyndall Effect in Quartz
The so-called Angel Feather Quartz is caused by the Tyndall Effect.
Watch the video
The Tyndall effect is light scattering by particles in a colloid or in a very fine suspension. Blue light is scattered much more strongly than red light, so it is a blue colour that we see when white light is scattered. The Tyndall effect is seen when light-scattering particulate matter is dispersed in an otherwise light-transmitting medium, where the diameter of an individual particle is in the range of roughly 40 to 900 nm, that is somewhat below or near the wavelengths of visible light (400–750 nm).
For example, blue eyes appear blue due to Tyndall scattering in a translucent layer in the iris. Brown and black irises have the same layer except with more melanin in it, and the melanin absorbs light
(Wiki).
The Tyndall effect in quartz arises in rare cases when white light is passed through the crystal and becomes scattered by minute particles, formed during previous growth stages of the crystal. The crystal appears colourless and featureless in normal lighting conditions, but when a narrow beam of white light is passed through it, distinct feathery blue phantom features can be seen. This is particularly dramatic when the quartz has been shaped into a crystal sphere.
Images
Localities for quartz
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Quartz is a primary silicate that occurs in wide veins
commonly intergrown with magnetite. Excellent crystals of milky
quartz
up to 4.5 cm long have been found, some coated with chlorite and
magnetite
(AJM 22.1.35).
At the Manuka Mine, Mouramba county, New South Wales, Australia, many large smoky quartz crystals were
discovered
in the
copper-lead-silver
prospect, and they exhibit a wide variety of forms, and both Dauphine and Japan law twins. The quartz crystals occur
in two different lithographies.
The first is in cavities up to 20 cm across in fossiliferous limestone,
as crystals that are likely to have formed during a
galena-sphalerite-acanthite-quartz
hydrothermal mineralisation event.
The second is in leached saprolitic
clay overlying dolomitised
limestone, in calcium-rich bands within the
clay
(AJM 22.2.55).
At Bahia, Brazil, quartz pseudomorphs after
talc have been found
(KL p254).
At Irai, Brazil, quartz pseudomorphs after
calcite have been found
(KL p247).
At the Rock Candy mine, British Columbia, Canada, quartz pseudomorphs
after fluorite have been found
(KL p251).
At Kitwanga, Skeena, British Columbia, Canada, acicular quartz has been found. This is a very rare habit for
quartz, but a road cut near Kitwanga has produced excellent examples of these crystals. They occur in
quartz veins cutting through a dark sandstone that is
brecciated and extensively replaced by quartz. The acicular
crystals are associated with prismatic quartz of normal proportions that are up to 8 mm long, massive
calcite and corroded calcite
crystals, microcrystals of anatase,
brookite, a clay mineral in the
kaolinite group, pyrite, and
spheres of a black manganese mineral. The acicular quartz was the
first mineral to form, and crystals can be found protruding from quartz of normal morphology that grew around
them. The acicular quartz crystals are 1 to 3 mm in length and as thin as 0.05 mm or less. Ratios of width to
length of 1:50 have been observed. These slender crystals are easily bent; the distortion is elastic and when pressure
is released, the crystal springs back to its original form. When the elastic limit is exceeded, the crystal breaks
free with great energy and disintegrates. This scale-dependent behaviour is shown by acicular crystals of many
minerals such as cuprite, halite and
natrolite, but is not often observed
(R&M 97.6.566-567).
At the Lyndhurst area, Ontario, Canada, strange crystals of sceptre quartz have been found, often with the cap
displaced from the trunk
(R&M 97.3.254-259).
The amethyst mines of the Thunder Bay area, Ontario, Canada, are the source of of some large groups of crystals of quartz
variety amethyst. Some are of the typical purple amethyst colour, and some have a distinctly red colour, due to
sub-surface inclusions of
hematite
(R&M 94.4.306-341).
In the vicinity of the Discovery Bay lookout, Lantau Island, Islands District, New Territories, Hong Kong, China, there
are several hydrothermal quartz veins and rare vugs with quartz crystals. Some of the massive quartz
of the vein shows very pale amethyst colour
(Mindat).
At Kwun Yum Shan, Yuen Long District, New Territories, Hong Kong, China, the deposit is a hydrothermal deposit which lies
along a fault zone withi altered acid volcanic rocks, consisting mainly of
chlorite, biotite,
sericite and actinolite with
scattered quartz
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
There are several “hot pots” near the top of
the hill. These hot pots were thought to be outlets of warm and moist air, which is heated below the ground and ejected
through fissures and cracks in the rocks. The rocks here, however, are more likely to be
pyroclastic in nature. Mineral veins of quartz,
pyrite and galena can be identified, and
large crystals of quartz are present in the rock. The Hong Kong Geological Survey has now re-interpreted the rock as an
altered intrusive rhyolitic
hyaloclastite. It is possible that the outcrop marks a vent feeder
of volcanic rocks
(Geological Society of Hong Kong newsletter 14.1).
At Lantau Peak, Lantau Island, Islands District, New Territories, Hong Kong, China, quartz veins with small crystals have
been found about halfway along the path between Mui Wo and Sunset Peak, at latitude 22.25994 and longitude 113.97927
(Mindat).
At Tongbei, Fujian province, China, pseudomorphs of opal variety
hyalite after quartz have been found with
spessartine
(KL p259).
At Cabiche, Quípama, Departamento de Boyacá, Colombia, quartz with bright yellow tips coloured by fibrous inclusions of
an apparently amorphous mineral have been found. The fibres have not to date (May 2019) been identified unambiguously, but
one possibility is halloysite
(R&M 94.3.240-247).
At Göpfersgrün, Wunsiedel, Wunsiedel im Fichtelgebirge, Upper Franconia, Bavaria, Germany, talc
pseudomorphs
after quartz have been found
(KL p234).
At Idar-Oberstein, Birkenfeld, Rhineland-Palatinate, Germany, quartz pseudomorphs after
calcite have been found
(R&M 95.3.275).
At the Porretta quartz sites, Porretta Terme, Alto Reno Terme, Metropolitan City of Bologna, Emilia-Romagna, Italy, the geological setting
is deep marine turbiditic sequences characterised by a rhythmic alternation of sandstone,
siltstone, clay and
marly beds.
Fractures in the sandstone are characterised by the presence of
calcite and quartz crystals. The quartz crystals typically show skeletal growth
forms and have inclusions of light yellow-grey clay. They frequently also contain water and
gaseous hydrocarbon inclusions. The crystals have smooth and lustrous, in part curved, faces and often form aggregates of two or more
crystals, usually in parallel growth. The quartz crystals are found everywhere in rock clefts of the Porretta
sandstone formation.
In window quartz (skeleton quartz) the edges grew more quickly than the faces, so the edges stand out like the frames of a window.
Crystals that grow very quickly often develop these skeletal growth forms; other examples are gold
and halite crystals. The faces on a skeleton quartz crystal grow from the edges to the
centre. Sometimes these faces grow as thin transparent plates, and if these plates are finally completed, the watery solution inside will
be trapped behind a "window".
Window quartz crystals from this locality are considered to be amongst the best in the world
(Mindat, QP, AJM 22.2.13-25).
At Charcas, San Luis Potosi, Mexico, quartz pseudomorphs after
danburite have been found
(KL p250, R&M 95.3.275).
At the Berg Aukas mine, Grootfontein, Otjozondjupa Region, Namibia, quartz is a monor constituent in both the country rock and the
orebodies. Drusy quartz is most commonly associated with descloizite and, together with
calcite, dolomite,
goethite and willemite, can form the matrix of some
descloizite crystals
(R&M 96.2.132).
At the Nikolaevskiy mine, Dalnegorsk, Russia, quartz pseudomorphs after
beta-quartz have been found. Also at Dalnegorsk, quartz pseudomorphs
after fluorite have been found
(KL p246, 252).
At Mkobola district, Mpumalanga province, South Africa, a chalcedony
pseudomorph after a quartz Japanese
twin has been found
(KL p258).
At the Witwatersrand Goldfield, South Africa, quartz is the most common mineral, constituting the bulk of the
quartzite and the matrix of the
conglomerate; it is also the most common mineral forming the
conglomerate clasts. The latter are primarily white to grey vein
quartz pebbles although less common quartzite and
chert pebbles also occur. Quartz is found associated with
baryte, calcite,
chalcopyrite, clinochlore,
dolomite, epidote,
galena, gold,
pyrite,
pyrobitumen, pyrochlore and
pyrrhotite. Several gold mines have
produced aesthetic quartz crystals, typically transparent
(R&M 96.4.341-342).
At the Welkom goldfield, Lejweleputswa District, Free State, South Africa, large specimens of quartz weighing
several kilograms have been found
(R&M 96.4.341-342).
At the President Brand Mine, Lejweleputswa District, Free State, South Africa, quartz crystals are partially to wholly
coated by a thin film of light green clinochlore producing a glittering sheen.
Clinochlore can also be included within some
crystals imparting an attractive green colour to the quartz. Doubly terminated crystals have also been found here
(R&M 96.4.341-342).
At the Mponeng Mine, West Wits, Far West Rand, West Rand District Municipality, Gauteng, South Africa, well formed
quartz crystals to 30 cm, often as doubly terminated floaters, were common
(R&M 96.4.341-342).
At the Kusasalethu Mine, Carletonville, Western Sector, Far West Rand, West Rand District Municipality, Gauteng, South Africa,
noteworthy transparent crystals of quartz have been found
(R&M 96.4.341-342).
At the Kopanang Mine, Klerksdorp, Dr Kenneth Kaunda District Municipality, North West, South Africa, a fault was
intersected that yielded quartz specimens that are typically elongated prisms, transparent, and associated with
brown, goethite-altered pyrrhotite and
pyrite. Quartz specimens associated with grey rhombohedral
calcite display distinct growth features via internal zoning, producing phantom
forms
(R&M 96.4.341-342).
At the Buffelsfontein gold mine, Stilfontein, Dr Kenneth Kaunda District, North West, South Africa, noteworthy transparent
crystals of quartz have been found
(R&M 96.4.341-342).
At Asar hill, Güğtı, Dursunbey district, Balikesir province, Marmara region, Turkey, quartz
pseudomorphs after calcite have
been found
(KL p248).
At Wheal Mary Ann, Menheniot, Cornwall, England, UK, quartz pseudomorphs after
fluorite have been found
(R&M 95.3.275).
At the Fall Hill quarry, Ashover, Derbyshire, England, UK, quartz crystals occur on
fluorite
(RES p104).
At Calton Hill quarry, Buxton, Derbyshire, England, UK, quartz variety amethyst has been found lining a cavity in
basalt
(RES p116).
At Diamond Hill, Buxton, Derbyshire, England, UK, quartz variety eisenkiesel occurs in a
baryte
matrix
(RES p134).
At Waterswallows quarry, Buxton, Derbyshire, England, UK, quartz variety amethyst has been found in a vug in
dolerite
(RES p135).
At Kenslow Knoll sandpit, Newhaven, Derbyshire, England, UK, quartz crystals occur on
baryte
(RES p114).
At the Harry Stoke mine, near Filton, South Gloucestershire, England, UK, quartz variety amethyst has been found with
celestine and calcite
(RES p168).
At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, quartz is associated with analcime
and is often well crystallised and showing left-hand faces. A cavity has been discovered with walls lined by a film of
epidote with a little hematite, upon which was deposited a
comb structure of thousands of small quartz crystals up to 12 mm in length and up to 5 mm in cross section. Of 52 crystals examined, 43 showed
the development of the left trigonal pyramid {2111}, and rarely the left trigonal trapezohedron {6111}
(JRS 20.24-25).
At the Cloud Hill quarry, Breedon on the Hill, Leicestershire, England, UK, quartz crystals have been found with
hematite in a cavity in goethite, and
also as variety
eisenkiesel on a goethite matrix
(RES p206).
At the Bardon Hill quarry, Coalville, Leicestershire, England, UK, quartz crystals have been found encrusted with
dolomite
(RES p194).
At Peldar Tor quarry, Spring Hill, Whitwick, Leicestershire, England, UK, quartz crystals have been found
heavily included with
chlorite
(RES p201).
At the Snailbeach mine, near Minsterley, Shropshire, England, UK, quartz crystals coat both
calcite
rhombohedra and galena crystals. Quartz also occurs here with
cerussite
(RES p270, 272, 275, 276).
At the Edwin Richards quarry, Rowley Regis, Dudley, West Midlands, England, UK, quartz variety amethyst
has been found with
dolomite in a calcite vein
(RES p330).
At the Finch mine, Hayden, Banner district, Gila county, Arizona, USA, quartz
pseudomorphs after wulfenite
have been found
(KL p253).
At Black Canyon City, Maricopa county, Arizona, USA, large pseudomorphs of
quartz after both
anhydrite and aragonite have
been found (R&M 94.2.166-167, KL p245).
At the Potter-Cramer mine, Vulture Mining District, Maricopa county, Arizona, USA,
secondary colourless quartz crystals fill vugs and
fractures within the host rock. Surprisingly the quartz fluoresces bright bluish white under SWUV, but not MW
or LW, and it does not exhibit phosphorescence. It is not known what activates the fluorescence
(R&M 96.1.34).
At the South Belmont Mine, Santa Cruz county, Arizona, USA, fine epimorphs of quartz after
calcite have been found
(R&M 94.2.160).
In the South Comobabi Mountains, Pima county, Arizona, USA, epimorphs of
quartz after
calcite have been found, sometimes with
hematite inclusions (R&M 94.2.161).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, quartz can occur as crystals up to 25 cm long. Very dark
smoky quartz crystals tend to occur in late-stage secondary phosphate assemblages with
rhodochrosite. The Emmons pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.514).
The Central Mine, Central, Keweenaw county, Michigan, USA, initially targeted a series of sub-parallel mineralised
fissure veins where the most copper-rich portion of the vein was close to the
base of the main greenstone flow.
Quartz was an abundant gangue mineral in the fissure veins of the
Central mine, and where the vein was vuggy or brecciated, fine crystallized quartz specimens were sometimes
encountered, showing colourless and transparent, prismatic crystals up to 8 cm. Usually these quartz crystals
are associated with lime-green prehnite,
epidote, pumpellyite,
copper and rarely silver. More commonly,
quartz occurs as stubby 2 to 10 mm prismatic crystals which fill narrow open fractures in
basalt, or coat the walls of mineralised amgydules in the
basalt flows.
(MinRec 54.1.80-81)
At the Connecticut Mine, Delaware, Keweenaw county, Michigan, USA, microscopic quartz
pseudomorphs after microcline
have been found
(R&M 97.4.358).
The Cliff Mine, Phoenix, Keweenaw county, Michigan, USA, is situated at the base of a roughly 70-metre
basalt cliff. A curious feature of the impressive thickness of the
greenstone flow here is that it contains zones of “pegmatoid”: areas
where
slow cooling in the core of the lava flow allowed for large feldspar crystals
exceeding 1 cm to grow. Such features are normally only observed in intrusive igneous rocks and are almost unheard of
in basalt flows.
The Cliff mine primarily exploited rich copper mineralisation in the Cliff
fissure (vein). Although mineralised with copper to some extent along its
entire length, the part of the vein just below the greenstone flow
carried the richest copper mineralisation by far. A significant amount of the
copper recovered at the Cliff mine came from amygdaloids in the tops of 13
basalt flows which were cut by the Cliff vein. The discovery and mining
of this vein proved that the veins were the source of the large masses of float
copper that were already well known, and proved that the
primary ore mineral in the district was native
copper, not sulphides, as had been suspected earlier.
Quartz as white or colourless and transparent crystals is common in the Cliff vein. Fine examples in association
with prehnite and other minerals are known. As for all colourless species
occurring at the Cliff mine, copper inclusions in
quartz crystals are well documented
(MinRec 54.1.25-49).
At the Little Gem amethyst mine, Jefferson county, Montana, USA, quartz occurs in a
pegmatite outcrop hosted in
biotite-hornblende
granite and containing pockets of significant size, mostly enclosed
partially or completely within
microcline feldspar. Pockets in the core of the
pegmatite contain only quartz. Elsewhere they contain
quartz and
microcline, but seldom albite, with
occasional minor quantities of epidote,
schorl, anatase,
pyrite (typically altered to limonite)
and goethite.
Quartz occurs in a number of varieties, including milky quartz, rock crystal, smoky quartz
and amethyst. Amethyst is a
late stage generation of quartz, often the last, exhibiting habits including sceptres and orientated overgrowths. The
purple colour is caused by trace quantities of iron as Fe3+, so where there is insufficient
iron
rock crystal or smoky quartz form in preference to amethyst.
Sceptres are very common, generally with milky or smoky stems and amethyst heads.
Jacaré or crocodile quartz has overgrowths that crystallised from the hydrothermal fluids that created
secondary minerals. The substrate is milky to
smoky quartz, and the jacaré is usually amethyst.
Many of the quartz crystals have fluid inclusions (R&M 93.6.498-516).
At the PC Mine, Cataract Mining District, Jefferson county, Montana, USA, quartz occurred as single crystals, groups, and
as Japan-law twins. Specimens are mostly colourless, rarely smoky. Individual crystals are up to 30 cm long and groups up to 76 cm.
Inclusions include anhydrite, hematite,
sericite, schorl and
pyrite. It is estimated that more than four thousand specimens of Japan-law twins were found
here.
A twin is the symmetrical intergrowth of two crystals. A Japan-law twin is a contact twin with the c-axes inclined to each other at
an angle of 84.55 degrees. A Reichenstein-Grieserntal twin is a rare contact twin with the inclination of the c-axes at 76.43
degrees. Only five are known to have been found at the PC mine
(R&M 96.490-501)
In Nebraska, USA, quartz pseudomorphs after
gypsum have been found
(KL p249).
At Cookes Peak mining district, Luna county, New Mexico, USA, quartz occurs as massive replacement of
limestone bodies, and as crystals lining cavities in these bodies. It
is occasionally associated with fluorite, and is sometimes found as quartz
epimorphs after fluorite. Excellent epimorphs of quartz after
calcite have also been found, some associated with
fluorite (R&M 94.3.234-235).
At the Dafoe property, Pierrepont, St. Lawrence county, New York, USA, quartz
occurs as primary, Tessin-habit crystals to 15 cm and is found in
association with tourmaline. Crystals are normally white to grey, poorly
formed, and often coated with white talc.
Secondary, or late-stage, quartz occurs as fine, prismatic,
transparent crystals to 18 cm. It also is found as attractive molds after
calcite, tourmaline and an
undetermined species, likely baryte
(R&M 97.3.250).
At Rose Road, Pitcairn, St. Lawrence county, New York State, USA, quartz occurs at the
skarn
deposit as pseudomorphs after wollastonite, either as isolated
crystals in areas of coarsely crystallised
calcite or as crystals lining the walls of a
diopside-albite rock that faces into
coarsely crystallised calcite
(R&M 97.5.434-444).
At the Devil's Den locality, Mount Tabor, Rutland county, Vermont, USA, collecting is no longer permitted, but in the
past it has been a prolific source of various varieties of quartz, including faden quartz, smoky quartz and
Mount Tabour "twins".
The "twins" are pairs of parallel crystals that share a common prism face and a pair of terminations with a
church-and-steeple appearance (R&M 94.3.266-272).
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