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Formula: KLiMn3+2Si4O10O2
Phyllosilicate (sheet silicate), trioctahedral mica group,
mica group,
lithium- and manganese- bearing
mineral
Crystal System: Monoclinic
Specific gravity: 3.264 measured, 3.255 calculated
Hardness: 2½
Streak: Light brown
Colour: Black to brownish black
Solubility:
Common impurities: Ti,Al,Fe,Mg,Ba,Na,F,Cl,H2O
Environments
Norrishite is a major to minor component of manganiferous
schists, formed through metamorphism of a stratiform
manganese deposit. Resembles
biotite
(Webmin).
Localities
At the type locality, the Hoskins mine, Grenfell, Forbes county, New South Wales, Australia, norrishite has been
found in several manganese silicate-oxide rock samples, occurring as a
scatter of small dumped rocks near the main shaft of the mine. At this location, a steeply dipping stratiform unit of
metamorphosed manganese-rich rocks, up to 8 m thick, is associated with
metamorphosed jasper, basalt
and siltstone. The rocks are oxidised to
manganese oxide assemblages to a depth of 45 m, and therefore, although not
found in situ, the unweathered primary norrishite-bearing
rocks probably came from below this depth.
Two major mineral associations are evident in the manganese-rich rocks:
(1) a "reduced" assemblage containing rhodonite,
tephroite, hausmannite,
garnet (spessartine -
grossular - andradite),
calcium-manganese carbonates, quartz,
manganoan magnetite,
manganese-rich chlorite and
baryte
(2) an "oxidised" assemblage containing a manganese-rich alkali
amphibole resembling
mangano-ferri-eckermannite,
manganese-rich alkali clinopyroxene,
manganoan
pectolite-serandite,
braunite, norrishite, calcium and
barium carbonates, albite,
K-feldspar, quartz and
baryte.
Norrishite, occurring as shiny black flakes up to 1.5 mm across, has been recognized from two contrasting
assemblages within the oxidised manganese silicate rocks:
(A) It is a major component, up to 25 modal %, of the assemblage amphibole
+ clinopyroxene + braunite +
manganoan pectolite +
norrishite +/- carbonate +/- feldspar. Individual laminations
containing >40 modal % norrishite attain 6 mm in thickness.
(B) It is a minor component (up to 5 modal %) of the rare assemblage quartz +
amphibole + clinopyroxene +
norrishite +/- pectolite +/- carbonate. Norrishite apparently
coexists stably with the other primary minerals, and no
replacement textures are recognised. Inclusions of amphibole,
clinopyroxene and braunite occur
sparsely in norrishite, which forms well crystallised platey grains
(AM 74.1360-1367).
It also forms centimetre-sized black aggregates in dark purple sugilite in
the oxidised hydrothermal manganese deposit. Associated minerals include
sugilite,
potassic-mangani-leakeite,
pectolite and hennomartinite
(Mindat).
At the Wessels Mine, Hotazel, Kalahari manganese field, Northern Cape, South Africa, norrishite, an OH-free
mica, is found in a silicate-rich sample. The mineral forms centimeter-sized
black aggregates in dark purple sugilite, and is associated with
potassic-mangani-leakeite and manganese-rich
pectolite. It is suggested that the hydrothermal activity is related to the
collision and tectonism affecting the edges of the Kalahari craton
(AM 88.189-194).
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