Rankinite

rankinite

larnite

melilite

kilchoanite

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Formula: Ca₃Si₂O₇
Sorosilicate (Si₂O₇ groups), paramorph of kilchoanite
Crystal System: Monoclinic
Specific gravity: 2.96 to 3.00 measured, 3.00 calculated
Hardness: 5½
Streak: White
Colour: Colourless
Solubility: Gelatinises readily in weak hydrochloric-sulphuric acid solutions, precipitating abundant gypsum (Mindat)
Environments

Metamorphic environments

Rankinite is formed by high temperature contact metamorphism of impure limestone in calc-silicate skarn. Associated minerals include larnite, melilite, spurrite, kilchoanite, nagelschmidtite, gehlenite, cuspidine, monticellite, andradite, perovskite and magnetite (HOM).

Localities

At the type locality, Scawt Hill, Larne, County Antrim, Northern Ireland, UK, rankinite is found in two associations.
(a) as rounded or irregular grains in melilite rocks, associated with larnite or wollastonite
(b) at the contact of flint nodules in metasomatised limestone, as a narrow zone of crystals separating wollastonite and larnite (AM 27.720).

Alteration

kilchoanite to rankinite
Ca6(SiO4)(Si3O10) ⇌ 2Ca₃Si₂O₇
Higher temperatures favour the forward reaction (MM 34.1.1-16).

wollastonite and spurrite to rankinite and CO₂
4CaSiO₃ + Ca₅(SiO₄)₂(CO₃) ⇌ 3Ca₃Si₂O₇ + CO₂
Higher temperatures favour the forward reaction (MM 34.1.1-16).

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