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Formula: ([Hg1+]2)3OCl3(OH)
Hydroxyhalide, forms a series with kadyrelite
Crystal System: Isometric
Specific gravity: 8.33 to 8.45
Hardness: 2½
Streak: Yellow to greenish yellow
Colour: Yellow, orange-yellow, brown, turns dark brownish, then black, on exposure to light, yellow-brown to brown
in transmitted light
Solubility: Decomposed by acids with separation of calomel
Environments
Sedimentary environments
Hydrothermal environments
Eglestonite is an oxidation product of other mercury minerals such
as native mercury and cinnabar
(Mindat).
Localities
At the Kadyrel' Hg occurrence, Oorash-Khem River Valley, Uyuk Range, Pi-Khem District, Tuva, Russia, eglestonite
occurs with lavrentievite,
calomel, mercury and
kadyrelite
(HOM).
At the Mariquita Mine (Sultana Mine), Usagre, Badajoz, Extremadura, Spain, eglestonite is the most significant
supergene mercury
mineral, both for its abundance and for the high quality of the recovered specimens. It has been found in a variety
of habits.
In recently fractured areas, eglestonite is usually found as yellow to orange earthy crusts and masses.
Sometimes the crusts are rounded, possibly as a result of the oxidation of drops of
native mercury, leaving a
pseudomorphic remnant of eglestonite shaped like a hollow
droplet.
The largest masses of eglestonite have been found as inclusions in
baryte; these are compact aggregates to several centimetres across, of a
vivid yellow to orange colour.
Well-crystallized eglestonite occurs in cavities several centimeters wide, filled partially or completely with
native mercury.
Crystals of eglestonite up to 4 mm in size are sometimes found in druses which cover areas of
calcite or baryte matrix to
10 x 10 cm2. Frequently, long-prismatic or acicular crystals rise from the dodecahedral faces; these
crystals can reach 1 cm long, although most do not exceed 2 or 3 mm.
Occasionally chocolate-brown dodecahedral crystals of eglestonite up to 1 mm in size have been found
associated with montroydite.
In some cavities large masses of vivid orange eglestonite have been found associated with large tabular
crystals of calomel on highly altered cleavable
calcite; some of these may be eglestonite
pseudomorphs after calomel
(MinRec 55.4.498-502).
Eglestonite from the Mariquita Mine -
Image
At the Funderburk prospect, Pike county, Arkansas, USA, eglestonite occurs
in fractures in sandstone with artsmithite and other
mercury minerals
(Minrec 35.4.344).
At the Clear Creek claim, Goat Mountain, New Idria Mining District, San Benito county, California, USA,
eglestonite is most often associated with edgarbaileyite,
native mercury, cinnabar and
wattersite coating quartz cavities
(Minrec 36.4.346-347).
Eglestonite from Clear Creek - Image
At the Challenge deposit, Redwood City, San Mateo county, California, USA, eglestonite is associated with
cinnabar, mercury and
calomel
(HOM).
Eglestonite from the Challenge Deposit -
Image
At the type locality, the Terlingua Mining District, Texas, USA, eglestonite is associated with
calomel, mercury,
terlinguaite, montroydite
and calcite
(HOM).
Eglestonite from Terlingua - Image
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