Rankinite

minerals

Formula: Ca3Si2O7
Sorosilicate (Si2O7 groups), paramorph of kilchoanite
Specific gravity: 2.96 to 3.00
Hardness: 5½
Streak: White
Colour: Colourless
Solubility: Gelatinizes 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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