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Formula: CrO(OH)
Oxide hydroxide mineral, paramorph of
bracewellite and grimaldiite,
chromium-bearing mineral
Crystal System: Orthorhombic
Specific gravity: 4.67 calculated
Streak: Yellow-brown, greenish brown
Colour: Reddish brown, golden brown, greenish brown, green
Common impurities: Fe,Al,V
Environments
Localities
At the Outokumpu mining district, Outokumpu, North Karelia, Finland, Guyanaite occurs in
chromium-rich tremolite
skarn, metaquartzite,
and chlorite veins
(HOM).
At the type locality, the Merume river, Kamakusa, Potaro-Siparuni Region, Guyana, guyanaite occurs as
microcrystalline aggregates and alluvial grains, and very rarely as prismatic crystals, up to 0.1 mm in size, in
an alluvial placer deposit. The guyanaite forms fine-grained intergrowth with other
chromium oxide-hydroxides Associated minerals include
quartz, pyrophyllite,
mcconnellite, grimaldiite,
gold, gahnite,
eskolaite and bracewellite
(Mindat, HOM).
At Moses Rock, San Juan County, Utah, USA, guyanaite has been identified in a xenolith of
chromium-rich omphacite
from the Moses Rock diatreme in the Navajo Volcanic Field. It occurs as the dominant phase in small clusters of
accessory minerals, intergrown with kosmochlor-rich
omphacite, zincochromite,
eskolaite and carmichaelite.
The assemblage is interpreted as the result of metasomatism
of chromite-bearing
serpentinite by slab-derived fluids during subduction. At the
time of entrainment of the xenolith the rock was undergoing
prograde metamorphism, with guyanaite dehydrating
to eskolaite plus water
(AM 99.1277-1283).
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