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Formula: Pb2(UO2)(AsO4)2.nH2O
Hydrated normal arsenate, uranyl
mineral, forms a series with
parsonsite, and forms oriented intergrowths with
hügelite (Dana)
Crystal System: Triclinic
Specific gravity: 6.39 measured, 6.40 calculated
Hardness: 2½ to 3
Streak: Light yellow
Colour: Yellow
RADIOACTIVE
Environments
Sedimentary environments
Hydrothermal environments
Hallimondite is a rare mineral that was first discovered on a museum specimen from an oxidising
arsenic-lead bearing deposit, formed by
alteration of galena
(HOM).
Localities
At the type locality, the Michael Mine, Weiler, Seelbach, Seelbach, Ortenaukreis, Freiburg Region, Baden-Württemberg,
Germany, hallimondite occurs as finely crystalline yellow material in druses of
chert breccia, associated with
hügelite and mimetite, rarely as
crystals up to 0.3 to 0.4 mm on hügelite. The hallimondite occurs in
the oxidation zone of an arsenic-lead bearing
deposit containing various sulphides and much native arsenic, along with
zeunerite, nováčekite,
metaheinrichite, kasolite,
mimetite, adamite,
hügelite, widenmannite,
weilerite, baryte,
galena and quartz
(AM 47.414-420, HOM).
Hallimondite is found as small crystals and fine-grained coatings in cavities and fractures of
quartz. It contains inclusions of baryte and
galena. Earthy mimetite in places coats
hallimondite aggregates, thus proving that hallimondite is older. On the other hand hallimondite
evidently was formed later than hügelite, for in one specimen crystals of
hallimondite were found as an incrustation on hügelite. The formation of
hallimondite and hügelite is due to the action of
arseniferous and uraniferous waters on
galena. The arsenic acid has been derived
by oxidation of native arsenic which is locally abundant. No
primary uranium minerals occur in the vein, so it is likely that
the uranium was derived from the wall rock
(AM 50.1143-1157).
Hallimondite from the Michael Mine - Image
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