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Formula: Fe(OH)3
Hydroxide, söhngeite subgroup,
perovskite supergroup
Crystal System: Orthorhombic
Specific gravity: 3.32 measured, 3.35 calculated
Hardness: 4
Streak: Pale green
Colour: Dark bottle-green to yellow-green; yellowish bottle-green in thin section
Common impurities: C,Pb,Si,Zn
Environments
Metamorphic environments
Hydrothermal environments
Localities
At the type locality, Broken Hill, Broken Hill district, Yancowinna county, New South Wales, Australia,
bernalite occurs as flattened pyramidal to pseudo-octahedral crystals up to 3 mm on edge with concretionary
goethite and coronadite. It was
found on a museum specimen from the metamorphosed lead -
zinc deposit, probably from the surface oxidation zone. The crystals are dark
bottle green, with a vitreous to adamantine lustre and a pale green streak.
The dominant minerals in the near-surface gossan are
coronadite and goethite, and
there are reports of lepidocrocite and
chalcophanite, but the existence of other
manganese and iron oxides or
hydroxides had not been fully investigated at the time of writing (1993).
The bernalite crystals contain small but significant amounts of lead,
silicon and zinc
(AM 78.827-834).
The Clara Mine, Oberwolfach, Ortenaukreis, Freiburg Region, Baden-Württemberg, Germany, has been worked as a
baryte - fluorite deposit and
consists of a system of hydrothermal veins hosted mainly by paragneiss.
Two samples of bernalite have been found on the dumps.
In sample A, found in 1993, the bernalite forms bright olive-green to dark bottle-green crystalline aggregates,
closely intergrown with brownish black goethite. Voids in mamillary
goethite host transparent, vitreous crystals up to ~0.1 mm, resembling
tetragonal prisms; the prisms are commonly intergrown either in a subparallel way or by forming a boxwork of crystals.
Both bernalite and goethite cover
milky quartz and colourless, slightly corroded
fluorite. Older, whitish baryte also
is present, but not in contact with the iron minerals.
Sample B, found in 1989, similarly has olive-green to yellowish green crusts of bernalite on mamillary, dark
brownish black goethite. Close inspection revealed fine-grained layers of
bernalite, less than 0.5 mm thick, between layers of goethite 1 to
5 mm thick, suggesting simultaneous growth of the two minerals. Rare, single, prismatic crystals of bernalite
up to 0.1 mm long have a square or slightly rectangular cross-section and a perfectly flat top
(CM 36.1211-1216).
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