Formula: Na4[Fe2+10Ti2]O4[Si12O36]
Inosilicate (chain silicate), titanium-bearing mineral
Specific gravity: 3.74 to 3.85
Hardness: 5½ to 6
Streak: Reddish brown
Colour: Velvet-black
Solubility: soluble in HF
Common impurities: Al,Mn,Mg,Ca,K,Cl
Environments:
Plutonic igneous environments
Volcanic igneous environments
Aenigmatite is a rock-forming mineral, relatively common in sodium rich alkaline rocks. It occurs in
nepheline and sodalite
syenites, and occasionally in alkali
granite.
It also occurs in phenocrysts and
in the groundmass of alkaline lavas, such as trachyte.
Common associates include aegirine,
arfvedsonite, augite,
fayalite, hedenbergite,
ilmenite and riebeckite
Alteration
aenigmatite, anorthite and O2 to
hedenbergite, albite,
ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] +
CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 +
2NaAlSi3O8 + Fe2+Ti4+O3 +
Fe2+Fe3+2O4
(DHZ 2A p651)
titanomagnetite (ilmenite combined with
magnetite),
quartz, and
aegirine-hedenbergite to
aenigmatite, hedenbergite,
magnetite and O2
6(Fe2+Ti4+O3 + Fe2+Fe3+2O4) +
12SiO2 + 12(NaFe3+Si2O6 + CaFe2+Si2O6)
⇌
3Na4[Fe2+10Ti2]O4[Si12O36] +
12CaFe2+Si2O6 + 2Fe2+Fe3+2O4 +
5O2
(DHZ 2A p652)
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