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Formula: Mg0.5NiFe3+(Te4+O3)3.4H2O
Tellurite,
zemannite group,
nickel-bearing mineral
Crystal System: Hexagonal
Specific gravity: 4.102 calculated
Hardness:
Streak: Pale yellow-green
Colour: Golden yellow
Luminescence: Not fluorescent
Environments
Keystoneite was first identified from a specimen of "ferrotellurite", a mineral with the reported formula
Fe2+Te6+O4. The discreditation of "ferrotellurite" has been accepted by the
IMA-CNMNC, as no material corresponding to a phase remotely similar to
Fe2+Te6+O4 was found on any historical samples labelled as containing
"ferrotellurite"
(Mindat).
Localities
The type locality is the Keystone Mine, Magnolia Mining District, Boulder County, Colorado, USA. Light yellow-green
crystals of keystoneite were identified on a freshly split fracture surface on a "ferrotellurite" specimen
traceable to 19th century mineralogist Frederick A. Genth. Associated minerals on the sample are
calaverite, magnolite,
native mercury, melonite,
paratellurite, pyrite,
quartz, stibnite and
native tellurium. Keystoneite is likely to originate from aqueous
weathering of melonite and
tellurium-rich pyrite, followed
by crystallisation in the presence of magnesium-rich fluids.
Keystoneite forms light yellow to light green hexagonal prismatic crystals, often in clusters, sprays, or
parallel growth aggregates. Individual crystals rarely exceed 200 microns in length and 20 microns in
cross-section. The streak is light yellow-green and the lustre is adamantine
(CM 59.355-364).
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