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Formula: NaFe2+Fe3+Si6O15
Inosilicate (chain silicate) with double chains, tuhualite group
Crystal system: Orthorhombic
Specific gravity: 2.89 measured, 2.86 calculated
Hardness: 3 to 4
Colour: Dark blue to black, violet
Common impurities: Ti,Zr,Al,Mn,Mg,Ca,F,Cl,H2O, Na may be replaced by minor K
Environments
Tuhualite occurs as crystallisation from magma quite impoverished in lime
due to early fractionation of
aegirine-hedenbergite, and
perhaps aegirine-augite, in the
intratelluric (situated, formed, or occurring deep within the earth) stage
(Mindat).
Localities
At Pantelleria Island, Trapani Province, Sicily, Italy, an occurrence of tuhualite is reported in a peralkaline
rhyolitic ignimbrite. The
mineral formed during devitrification of glassy fiamme (lens-shapes, usually millimeters to centimeters in size, seen
on surfaces of some volcaniclastic rocks) and varies in form from anhedral plates to
perfectly euhedral crystals in open vesicles. There is significant replacement of Fe2+ by Mn and Zn, and of
Na by K. The tuhualite possibly formed at temperatures less than 580°C in the presence of a hydrous fluid
(EJM 30.2.367–373).
At the type locality, Mayor Island, Bay of Plenty Region, New Zealand, tuhualite occurs as a
primary igneous mineral and in vesicles of some silicic lavas, as
rhyolite. Associated minerals include
alkali feldspar, quartz,
aegirine, riebeckite,
aenigmatite and arfvedsonite
(HOM).
Tuhualite from Mayor Island
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