Formula: Ca2(SiO4)
Nesosilicate (insular SiO4 groups), olivine group, the high-temperature, monoclinic
paramorph of calcio-olivine, which is orthorhombic.
Specific gravity: 3.28 to 3.33
Hardness: 6
Streak: White
Colour: White, grey, colourless in thin section
Solubility: Gelatinised by dilute hydrochloric acid
Common impurities: Al,Fe,Mg,Na,H2O,C,P
Environments
Larnite occurs in metamorphosed sedimentary carbonate formations in limestone or
chalk in contact with molten
basaltic rocks. Larnite is a high temperatures mineral stable in the range of 520o
to 670oC; at lower temperatures it is metastable and inverts to its low-temperature polymorph,
calcio-olivine, when subjected to mechanical shock. Common associates include
brownmillerite, chlormayenite,
gehlenite,
grossular-hibschite series minerals,
melilite group minerals, merwinite,
perovskite, rankinite,
scawtite, spurrite and
wollastonite
(Mindat).
Localities
At the Hatrurim Basin, Tamar Regional Council, Southern District, Israel, larnite is associated with
brownmillerite, mayenite,
melilite and spurrite
(HOM).
At Tokatoka, Kaipara District, Northland Region, New Zealand, larnite is associated with rankinite,
kilchoanite, scawtite and
hydrogrossular
(HOM).
At the type locality, Scawt Hill, Larne, County Antrim, Northern Ireland, UK, larnite is found in the
contact zone of
chalk and dolerite associated with
calcite, spurrite,
melilite, gehlenite,
merwinite, spinel,
perovskite and wollastonite.
The chief assemblages of the contact zone are (a) spurrite (b)
spurrite-larnite (c)
spurrite-larnite-melilite (d)
spurrite-melilite-merwinite-spinel_larnite,
all with calcite
(AM 14.338-339).
At 'S Airde Beinn, Isle of Mull, Argyll and Bute, Scotland, UK, larnite occurs in metamorphosed
calcitic amygdules
(Dana).
At the Marble Canyon Mine, Apache Peak area, Culberson county, Texas, USA, larnite occurs in the
contact zone of a
syenite-monzonite intrusion (Dana).
The mine is situated at an outcrop of five different types of igneous rock that are arranged in a concentric pattern. From the centre out they
are: (1) a coarse-grained syenite (2) a coarse-grained green
monzonite (3) a medium-grained grey monzonite
(4) a discontinuous narrow border of olivine gabbro and
(5) small rhyolite dikes which cut the other rock types.
The larnite occurs as rounded crystals that cut across boundaries of associated crystals of
merwinite, rankinite and
melilite. It is unstable at atmospheric temperature and pressure, especially when disturbed
mechanically as by scratching, or when struck with a hammer
(AM 51.1766-1774)
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