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Formula: Na3Ca(Si3Al3)O12S
Tectosilicate (framework silicate), sodalite group. Complete
solid solution with
nosean may exist, but is very limited with
sodalite (Dana).
Crystal System: Isometric
Specific gravity: 2.38 to 2.45 measured, 2.4 calculated
Hardness: 8 to 8½
Streak: Bright blue
Colour: Ultramarine, midnight blue, bluish green, green
Common impurities: Fe,Mg,K,H2O
Environments
Igneous environments
Metamorphic environments
Most lazurites analysed to date represent only sulphide-rich varieties of
haüyne
and not a separate species (Mindat), so the information presented below may be more applicable to
haüyne than to lazurite.
Lazurite occurs as a contact metamorphic mineral in
marble and skarn, and at
pegmatite-limestone contacts, Also in some
syenite and alkaline volcanics (Dana, HOM).
Associated minerals include calcite,
pyrite,
diopside, humite,
forsterite, haüyne and
muscovite
(HOM, Mindat).
Localities
At the type locality, Ladjuar Medam, Sar-e Sang, Koksha Valley, Khash & Kuran Wa Munjan Districts, Badakhshan,
Afghanistan,
lazurite occurs in calcic skarn and
marble (Mindat ).
In the Koksha valley, lazurite pseudomorphs after
muscovite have been found
(KL p264).
At the Italian Mountain, Taylor Park, Dorchester Mining District, Gunnison county, Colorado, USA, lazurite
formed early, apparently in equilibrium with diopside,
forsterite and perhaps
barium-bearing phlogopite,
whereas grossular formed later.
Thomsonite represents a late-stage hydrothermal alteration of
lazurite. Thomsonite,
chlorite and hisingerite,
just beyond the lapis-lazuli zones, are also late minerals.
The deposits resulted from contact metamorphism of
subgreywacke and sS
dolostone by quartz
monzonite and quartz
diorite of the Tertiary (66.0 to 2.6 million years ago) Italian
Mountain stocks. Sodium, sulphur,
barium, strontium, chlorine and
fluorine were introduced in aqueous magmatic fluids. Early baryte may have
localised later barium-bearing
phlogopite and lazurite. Late introduction of
iron produced the iron-rich silicates,
unusual in lapis-lazuli. Peak metamorphic conditions were
pressure ~250 bars and temperature ~600oC
(CM 18.59-70).
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