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Formula: Mn2+8Si6O15(OH,Cl)10
Phyllosilicate (sheet silicate), pyrosmalite group, forms a
series with
pyrosmalite-(Fe),
manganese-bearing mineral
Crystal System: Trigonal
Specific gravity: 3.06 to 3.19 measured, 3.14 calculated
Hardness: 4½
Streak: Brownish yellow
Colour: Light brown
Common impurities: As,Zn,Mg,Ca,H20
Environments
Pyrosmalite-(Mn) occurs in metamorphosed iron and
manganese ores
(Webmin).
Localities
At Broken Hill, New South Wales, Australia, pyrosmalite-(Mn) occurs in association with iron-bearing
johannsenite and calcium-poor bustamite
(AM 41.581-591). Also with rhodonite, manganese-bearing
fayalite, manganese-bearing hedenbergite and
galena
(HOM).
At the Geco Mine, Gemmell Township, Thunder Bay District, Ontario, Canada,
calc-silicates within the basal 4/2
copper zone of the mine contain
garnet, gahnite,
sphalerite,
ferro-actinolite, epidote,
quartz, biotite,
plagioclase, chlorite,
muscovite, K-feldspar and
pyrosmalite (with Mn/(Mn + Fe) ratio ranging from 0.21 to 0.61, and up to 3.9 weight % Cl).
The calc-silicates probably represent metasomatic
remobilisation of dispersed Ca (and Cl) from sea-floor hydrothermal alteration of
mafic to intermediate volcanic rocks. The
calc-silicates formed initially at about 600°C and 3 to 5
kbar pressure in a mildly reducing environment during
upper-amphibolite- to
granulite- facies
regional metamorphism, and were altered subsequently at lower
temperatures, less than 500°C
(Canadian Journal of Earth Sciences 29.7.1375–1388).
At the Takanosu mine, Japan, pyrosmalite-(Mn) occurs with spessartine, manganese-bearing
tremolite and quartz
(HOM).
At the type locality, the Sterling mine, Sterling Hill, Ogdensburg, Franklin Mining District, Sussex county, New Jersey, USA, pyrosmalite-(Mn)
occurs in veinlets in the franklinite ore of a metamorphosed stratiform
zinc deposit, associated with
friedelite, bementite and
willemite
(HOM, Dana, AM 38.755-760).
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