Images
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
Meteorites
Hercynite occurs in high grade metamorphosed ferruginous argillaceous sediments and in some
mafic
and ultramafic igneous rocks. Also in placers.
Localities
At Moorina, Dorset municipality, Tasmania, Australia, hercynite has been found in
cassiterite placers
(Dana).
The type locality is Poběžovice, Domažlice District, Plzeň Region, Czech Republic.
Hercynite from Poběžovice -
Image
At Le Prese, Poschiavo, Bernina Region, Grisons, Switzerland, hercynite has been found in
gabbro with corundum and
sillimanite
(Dana).
At Sweetwater Wash, Old Woman Mountains, San Bernardino County, California, USA, hercynite has been found
with ilmenite, anorthite,
staurolite and corundum
(Dana).
About 5 km NE of Colton, St. Lawrence County, New York, USA, hercynite has been found in a
pelitic paragneiss
with sillimanite, ilmenite
and rutile
(Dana).
At Whittles Emery Deposits, Pittsylvania County, Virginia, USA, hercynite has been found with
högbomite in emery
(Dana).
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)
Back to Minerals