Chemistry:Arsendescloizite

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Short description: Lead-zinc mineral
Arsendescloizite
Arsendescloizite-Mimetite-mz149a.jpg
Arsendescloizite (green) with mimetite (yellow), from Durango, Mexico
General
CategoryArsenate mineral
Formula
(repeating unit)
PbZn(AsO4)(OH)
Strunz classification8.BH.35
Dana classification41.5.1.9
Crystal systemOrthorhombic
Crystal classDisphenoidal
H-M symbol: 2 2 2
Space groupP21 21 21
Unit cell433.99
Identification
ColorPale yellow to deep green, greyish brown
Mohs scale hardness4
|re|er}}Sub-vitreous
StreakWhite
DiaphaneityTransparent
Specific gravity6.57
Density6.57
Optical propertiesBiaxial (−)
Refractive indexnα = 1.990
nβ = 2.030
nγ = 2.035
Birefringence0.045
PleochroismNone
2V angleMeasured: 30°
Calculated: 38°
Dispersionr > v
ExtinctionXYZ = bac
Ultraviolet fluorescenceNone

Arsendescloizite is a lead-zinc mineral, approved by the IMA in 1982.[1] It is an arsenate analog of descloizite. Its first description was published in 1982.[2]

Properties

Arsendescloizite belongs to the adelite-descloizite group. It is the arsenate analog of the lead-zinc vanadate mineral descloizite. It grows crystals that are platy to wedge-shaped.[1] Its composition consists of lead (48.35%), oxygen (18.67%), arsenic (17.48%), zinc (18.99%), and contains a negligible amount of hydrogen (0.24%).[3] Crystals are tabular on {001}. These tabular crystals tend to intergrow into rose-like shaped aggregates. Observed forms are {001}, {011}, {101} and {111}. Unlike descloizite, arsendescloizite has no trace of vanadium in its composition whatsoever, thus it became quickly obvious that the mineral was from a new species. From the composition alone, this mineral could have also been the lead analog of austinite or the zinc analog of duftite. The specimen Pete and Paul were studying was found on a matrix of a copper ore that mainly consisted of tennantite. When studying the specimen, they found no trace of calcium or copper in its formula. This was weird due to the fact that it occurs with the calcium-rich mimetite and was found on a copper ore.[2]

Occurrence

Arsendescloizite's type locality is at the Tsumeb mine, Namibia.[1] It has been reported subsequently from Cumbria, England.[4] Arsendescloizite is also found in Durango, Mexico. Arsendescloizite occurs in oxidized hydrothermal polymetallic deposits as a secondary mineral.[3] It is associated with chalcocite, quartz, mimetite, goethite and willemite. It comes in a sequence[clarification needed] of these minerals, arsendescloizite being between mimetite and goethite.[2]

See also

References