Aenigmatite

aenigmatite

arfvedsonite

nepheline

sodalite

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Formula: Na4[Fe2+10Ti2]O4[Si12O36]
Inosilicate (chain silicate)
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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