Sorosilicate (Si2O7 groups)
Specific gravity: 2.84
Colour: Colourless, white
Solubility: Effervesces in hydrochloric acid and forms a silica gel (Dana)
Tilleyite forms in the contact metamorphic zone between igneous rocks and
limestone, formed at low pressure and a high temperature (Webmin, HOM) of
800oC, in a sanidine facies environments. Tilleyite is
sometimes replaced by wollastonite,
vesuvianite, andradite variety melanite and
Associated minerals include merwinite, spurrite, gehlenite, vesuvianite, grossular, wollastonite, thaumasite, fluorite and calcite (HOM).
At the type locality, the Crestmore quarries, Riverside county, California, USA, tilleyite is found in the contact metamorphic zone where granodiorite and quartz monzonite have invaded limestone. The green colour of the matrix is due to the presence of merwinite. Streaks of vesuvianite associated with grossular, and veinlets of thaumasite, traverse the rock. The tilleyite is associated with wollastonite, and spurrite is present in limited amounts. Some hydromagnesite is also present, as an alteration product coating the surface of the blocks (AM 18.469-473).
tilleyite to spurrite and CO2
Ca5Si2O7(CO3)2 ⇌ Ca5(SiO4)2(CO3) + CO2
At higher temperatures favour the forward reaction (MM 34.1.1-16).
wollastonite and calcite to tilleyite and CO2
2CaSiO3 + 2CaCO3 ⇌ Ca5Si2O7(CO3)2+ CO2
Higher temperatures favour the forward reaction, and at even higher temperatures tilleyite alters to spurrite and CO2 (MM 34.1.1-16).
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