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Formula: Pb2(UO2)3(AsO4)2O2.5H2O
Valence: Pb2+2(U6+O2)3(As5+O4)2O2.5H2O
Hydrated arsenate,
phosphuranylite group,
uranyl mineral
Crystal System: Monoclinic
Specific gravity: 5.1 measured, 5.8 calculated
Hardness: 2 to 3
Streak: Light yellow
Colour: Brown, orange-yellow, yellowish brown
Very strongly RADIOACTIVE
Environments
Localities
At the type locality, the Michael Mine, Weiler, Seelbach, Seelbach, Ortenaukreis, Freiburg Region, Baden-Württemberg,
Germany, hügelite occurs rarely, filling cavities in chert
breccia, where it is associated with
hallimondite,
widenmannite, zeunerite,
mimetite and cerussite
(AM 47.414-420).
Formation of native arsenic in hydrothermal base metal deposits and related supergene U6+
enrichment
Native arsenic is an occasional ore mineral in some hydrothermal base metal
deposits. Its rarity is surprising, as arsenic is a common constituent of
upper crustal fluids.
Hence, the conditions of formation must be quite special to precipitate
native arsenic. At the Michael vein,
galena, sphalerite and
native arsenic are the most abundant ore minerals. The two important ore
stages comprise (1) galena - baryte
and (2) sphalerite -
native arsenic - quartz,
the latter with a general mineral succession of pyrite →
sphalerite ±
jordanite-gratonite solid
solution → galena → native arsenic.
The native arsenic-bearing mineralisation formed by cooling of an at least
130°C hot saline fluid accompanied by a reduction due to the admixing of a sulphide-bearing fluid. Thermodynamic
calculations reveal that for the formation of native arsenic, reduced
conditions in combination with very low concentrations of the transition metals
iron, cobalt and
nickel, as well as low sulphide concentrations, are essential. “Typical”
hydrothermal fluids do not fulfil these criteria, as they typically can contain significant amounts of Fe and
sulphide. This results in the formation of arsenides, sulpharsenides or As-bearing sulphides instead of
native arsenic. Very minor amounts of
pyrite, sulfarsenides, and
arsenides record the very low concentrations of Fe, Co, and Ni present in
the ore-forming fluid. High concentrations of aqueous Zn and Pb lead to early saturation of
sphalerite and galena that
promoted native arsenic precipitation by decreasing the availability of
sulphide and hence suppressing realgar formation. Interestingly,
native arsenic in the Michael vein acted as a trap for
uranium during
supergene weathering processes. Infiltrating oxidising,
U6+-bearing fluids from the host lithologies reacted under
ambient conditions with galena and
native arsenic, forming a variety of
U6+ (±Pb) arsenates such as
hügelite, hallimondite,
zeunerite, heinrichite or
novacekite, together with
uranium-free minerals like
mimetite or anglesite. Reduced
(hypogene) uranium
phases like uraninite were never observed
(AM 105.5.727–744).
Hügelite from the Michael Mine -
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