Hercynite

oxide

enstatite

Formula: Fe2+Al2O4 multiple oxide, spinel group
Specific gravity: 3.95
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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