Pyrosmalite-(Fe)

minerals

fayalite

greenalite

grunerite

Formula: Fe2+8Si6O15(OH)10
Phyllosilicate (sheet silicate), pyrosmalite group, forms a series with pyrosmalite-(Mn)
Specific gravity: 3.06 to 3.19
Hardness: 4½
Streak: Light green
Colour: Yellow-green to tan, rarely pinkish. Surface often grey, dark green or brown due to alteration Dana. Colourless in transmitted light (Webmin)
Common impurities: Al,Mg
Environments

Metamorphic environments

Pyrosmalite-(Fe) occurs intergrown with sulphides, formed during metamorphism of a stratiform lead-zinc deposit, and as a retrograde reaction product derived from clinopyroxene in saline fluid inclusions in contact metamorphic rocks. Associated minerals include fayalite, greenalite, galena, sphalerite, clinopyroxene, hornblende, grunerite, garnet, biotite, magnetite and apatite (HOM).

Localities

Pyrosmalite-(Fe) occurs in the sulphide-rich central part of the Pegmont stratiform lead-zinc deposit, 175 km southeast of Mount Isa, Queensland, Australia, and probably formed during prograde metamorphism (AM 73.933-934).

The type locality is the Bjelke mine, Nordmark Odal Field, Filipstad, Värmland County, Sweden.

Alteration

Pyrosmalite-(Fe) has been found in saline inclusions in magmatic quartz from altered granodiorite. On heating above 450oC with a Ca-enriched saline inclusion fluid, it reacts to form clinopyroxene. This suggests that clinopyroxene was originally present in the inclusions at high temperature, and then underwent retrograde reaction with the saline fluid, on cooling, to form pyrosmalite-(Fe). In its simplest form, the equilibrium reaction for this transformation is:

Chlorine-rich pyrosmalite-(Fe) and Ca2+ to hedenbergite (a clinopyroxene), Fe2+, Cl- and water
Fe8Si6O15[(OH)6Cl4] + 3Ca2+ (aq) = 3CaFeSi2O6 + 5Fe2+ (aq) + 4Cl (aq) + 3H2O

(AM 88.151-158)

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