Formula: Ca5Si2O7(CO3)2
Sorosilicate (Si2O7 groups)
Crystal System: Monoclinic
Specific gravity: 2.838 measured, 2.88 calculated
Hardness: 5
Streak: White
Colour: Colourless, white
Solubility: Effervesces in hydrochloric acid and forms a silica gel (Dana)
Environments
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
calcite (Dana).
Associated minerals include merwinite, spurrite,
gehlenite, vesuvianite,
grossular, wollastonite,
thaumasite, fluorite and
calcite
(HOM).
Localities
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).
Alteration
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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