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Formula: KSc2☐2Be3Si12O30
Cyclosilicate, osumilite group,
scandium-
and beryllium- bearing mineral, frequently in epitaxial growth with very thin
crystals of scandium-free milarite up to
~10–20 µm across (Mindat).
Crystal System: Hexagonal
Specific gravity: 2.614 calculated
Hardness: 6
Colour: Greyish white
Luminescence: No fluorescence observed under UV
Environments
Oftedalite is a relatively new mineral, approved in 2004 and to date (February 2023) reported only from the
type locality.
Localities
At the type locality, the Heftetjern pegmatite, Tørdal, Drangedal, Vestfold og Telemark, Norway, the
granite-cum pegmatite
suite was characterised as a member of the mixed NYF (niobium,
yttrium, fluorine) – LCT (lithium,
cesium, tantalum) family. The
pegmatite is poorly zoned;
it consists dominantly of amazonitic
K-feldspar,
microcline,
oligoclase,
cleavelandite, quartz (in part
smoky) and a variety of dark to white micas, from minor
biotite to zinnwalditic phases
enriched
in rubidium, cesium,
zinc and scandium. A thin border-zone is
followed by a zone of graphic intergrowth of feldspars and
quartz, a core-margin zone of coarse blocky texture with abundant
albite, and a quartz core. Accessory minerals
are concentrated mainly in the margin of the core.
Primary accessory phases include
beryl, bazzite,
spessartine, gadolinite-(Y),
cerium- and scandium- rich
epidote, tourmaline,
scandium-bearing helvine,
phenakite, cassiterite,
scandium-bearing ixiolite,
zircon, monazite-(Ce),
scandium-bearing minerals of the
pyrochlore group, yttrotantalite
and yttrium-bearing fluorite.
A late mineral assemblage encountered mainly in vugs and fractures comprises
bertrandite, bavenite,
cesium-bearing bazzite,
tin-bearing titanite,
rynersonite, yttrium-bearing
milarite, kainosite-(Y),
hellandite-(Y), calcium-bearing
hingganite-(Y), stilpnomelane,
thortveitite, cascandite,
kristiansenite and
scandiobabingtonite.
Oftedalite – scandium-bearing
milarite was found in two samples: one containing green
tourmaline, yttrium-bearing
milarite, bazzite and an unidentified
micaceous mineral, the other containing
thortveitite, bazzite,
kristiansenite and bertrandite
(CM 44.943-949).
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