Formula: Ca6.4[H0.6Si2O7]2(OH)2
Sorosilicate (Si2O7 groups)
Specific gravity: 2.94 calculated
Streak: White
Colour: White, colourless in thin section

Metamorphic environments
Hydrothermal environments

Killalaite is a secondary mineral in cavities and veins in hydrothermally altered and thermally metamorphosed limestone (Webmin, HOM).


At the type locality, Inishcrone, Killala Bay, Sligo County, Connacht, Ireland, the common primary mineral assemblages in the order of increasing distance from the contact of limestone with granite are:
gehlenite - larnite +/- spurrite,
larnite - spurrite +/- gehlenite,
spurrite - wollastonite - rankinite - tilleyite +/- cuspidine,
spurrite - calcite +/- cuspidine,
calcite - wollastonite,
calcite - grossular
calcite - quartz.
The primary mineral assemblages have been altered by hydrothermal activity. Scawtite and cuspidine are the main alteration products of the spurrite-bearing rocks, and killalaite occurs either with coarsely crystallised calcite and afwillite in veins and cavities, or as replacements of tilleyite. Both afwillite and killalaite were formed later than scawtite, cuspidine and tilleyite. Killalaite and afwillite formed before xonotlite. Spurrite, wollastonite and rankinite were formed at high temperature and very low pressure. The stability field of afwillite and killalaite lies between about 350° and 550° C at pressures ranging between about 500 to 3500 bars (MM 39.544-548).

At Carneal, Glenoe, County Antrim, Ulster, Northern Ireland, UK, killalaite is associated with larnite, magnetite, perovskite, spinel and spurrite (HOM).

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