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Formula: K(MgAl2)(Al5Mg)(BO3)3(Si6O18)(OH)3O
Cyclosilicate (ring silicate), tourmaline group,
boron-bearing mineral
Crystal System: Trigonal
Specific gravity: 3.081 calculated
Hardness: 7
Streak: White to very pale-brown
Colour: Pale brown to brown
Luminescence: Non-fluorescent under UV
Environments
Maruyamaite is a relatively new mineral, approved in 2013 and to date (June 2023) reported only from two
localities in the Kokchetav Massif.
It is found in high-pressure rocks, and supposed to originate near the peak pressure conditions of
ultra high pressure metamorphism, within the diamond stability field
(Mindat).
Localities
At the type locality, the Kumdy-Kol area, Kokchetav Massif, Zerendy, Akmola Region, Kazakhstan, maruyamaite
occurs
as core domains of chemically zoned tourmaline in ultra high pressure
diamond-bearing pelitic
gneiss, and consists mainly of
quartz, K-feldspar and
tourmaline, with minor amounts of
goethite, titanite,
zircon, phengite,
phlogopite, apatite,
chlorite, zoisite,
pumpellyite, graphite and
diamond.
Tourmaline occurs as anhedral to euhedral crystals up to 2 mm across with
strong chemical zoning. K decreases from core to rim and the mantle and the rim compositions correspond to K-bearing
dravite or oxy-dravite.
Microdiamond inclusions occur only in the K-dominant core of
tourmaline, ie maruyamaite, and in
zircon. On the other hand, flake
graphite and quartz occur in the
mantle and rim
(AM 101.355-361).
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