Torbernite is a popular mineral among collectors
who seek uranium
bearing minerals. Its square tabular crystals are distinctive,
but might
remind someone of the mineral wulfenite, if not for the green
color of
tornernite. Autunite is a structurally related mineral that
also forms
square tabular crystals, but they are not typically solid green
and
autunite is fluorescent in UV light.
The structure of torbernite is composed of phosphate tetrahedrons
linked
to uranium-oxygen groups that form distorted octahedrons. The
phosphates
and uranium groups lie in sheets that are weakly held together
by water
molecules. This structure produces the tabular habit, the one
perfect
direction of cleavage and the relative softness.
Torbernite can lose water and convert to a
different mineral called
meta-torbernite of the meta-autunite/meta-torbernite group of
minerals.
The change to meta-torbernite will often produce a pseudomorph.
A
pseudomorph is generally an atom by atom replacement of one
mineral's
chemistry to form another mineral. The process leaves the crystal
shape of
the lost mineral intact. pseudomorph means false (psuedo) shape
(morph).
In this case, the conversion is not so dramatic since it involves
only the
lose of a few water molecules and therefore a good pseudomorph
is likely.
The conversion is irreversible and ongoing and all collection
specimens of
a certain age are almost certainly partially to totally converted.
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The presence of torbernite crystal has been
used by prospectors as an
indicator of profitable uranium ore in the area. Fine torbernite
specimens
should be stored in a closed container to avoid water loss.
Remember, this
is a radioactive mineral and should be stored away from other
minerals
that are affected by radioactivity and human exposure should
be limited.
PHYSICAL CHARACTERISTICS:
Colors are various shades of dark to light green.
Luster is vitreous to pearly on the main pinacoid.
Transparency crystals can be transparent but more commonly are
translucent
to opaque.
Crystal System is tetragonal; 4/m 2/m 2/m
Crystal Habits include tabular square crystals dominated by
two pinacoid
faces. Crystals can look cubic, but the pearly luster only on
the pinacoid
faces gives the true symmetry away. Crystals can form in parallel
growths
giving a "stacked book" kind of look. Also as crusts,
micaceous, foliated
and scaly aggregates.
Cleavage is perfect in one direction.
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MORE PHYSICAL CHARACTERISTICS
Fracture is uneven.
Hardness is 2 - 2.5
Specific Gravity is approximately 3.2+ (slightly above average
for
translucent minerals)
Streak is a pale green.
Associated Minerals are autunite, uranophane, uranocircite,
meta-torbernite, uraninite and other uranium minerals.
Other Characteristics: radioactive and cleavage sheets are not
bendable
but are in fact brittle.
Notable Occurences include Cornwall, England; Mitchell Co., North
Carolina
and Utah, USA; Shaba, Zaire; Germany and France.
Best Field Indicators are color, crystal habit, non-fluorescence,
radioactivity, associations and brittle cleavage sheets.
Description provided by:
Tom G
of Windsor OHIO |
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