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Formula: Fe2+Al2O4
Multiple oxide, spinel subgroup
Picotite is a magnesium- and chromium- bearing variety of hercynite.
Chrompicotite is a variety of "picotite".
Crystal System: Isometric
Specific gravity: 4.39 calculated
Hardness: 7½
Streak: Dark greyish green to dark green
Colour: Dark blue-green, yellow, brown, black
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:
Plutonic igneous environments
Placer deposits
Metamorphic environments
Hercynite occurs in high grade metamorphosed ferruginous argillaceous sediments and in some
mafic
and ultramafic igneous rocks. Also in placers.
Alteration
enstatite-ferrosilite and
andalusite to Fe-rich cordierite
and spinel-hercynite
5(Mg,Fe2+)SiO3 + 5 Al2SiO5 →
2(Mg,Fe2+)2Al4Si5O18 +
(Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and
with a low calcium content, the association of andalusite with
enstatite-ferrosilite is excluded by
the above reaction
(DHZ 2A p134).
spinel-hercynite, sillimanite and
quartz to sapphirine
7(Mg,Fe2+)Al2O4 + 2Al2SiO5 + SiO2 →
4(Mg,Fe)1.75Al4.5Si0.75O10
(DHZ 2A p633)
staurolite, annite and O2 to
hercynite, magnetite,
muscovite,corundum,
SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3
(AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4
+ Fe2+Fe3+2O4 + KAl2
(AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O
(DHZ 1A p860)
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