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Formula: Ca4[Mg6Al6]O4[Si6B3Al3O36]
Inosilicate (chain silicate), rhönite group,
sapphirine supergroup,
boron-bearing mineral
Crystal System: Triclinic
Specific gravity: 3.42 to 3.52 measured, 3.47 calculated
Hardness: 6½ to 7
Streak: White
Colour: Pale yellow, blue-green, greyish blue
Solubility: Only slightly attacked by acids
Common impurities: Ti,Fe,Mn,Na,K,F,H2O,P
Environments
Serendibite occurs typically in contact metamorphosed
calc-silicate rocks, particularly gneiss,
in zoned metasomatic skarn between marble and
feldsathic rock, tonalite or
granulite. Typical associated minerals include
diopside, apatite,
calcite, clinopyroxene,
spinel, pargasite,
phlogopite and plagioclase, but
quartz is conspicuously absent. Tourmaline,
grandidierite and sinhalite may or may not
be directly associated
(Dana). Other associated minerals include scapolite,
calcite, tremolite, microcline variety
hyalophane, uvite,
clinozoisite, forsterite,
warwickite and graphite
(HOM).
Localities
At Nunavut, Canada, serendibite occurs in skarn which formed between
marble and granite on the Melville
Peninsula, District of Franklin. The serendibite is associated with augite variety fassaite,
uvite-rich tourmaline,
clinozoisite, spinel and
calcite. Comparison with other occurrences suggests that the stability field of
serendibite may be restricted to silica-undersaturated bulk composition
(CM 15: 108-112).
At the Portage-du-Fort area, Pontiac RCM, Outaouais, Quebec, Canada, serendibite crystals occur exclusively in a
calc-silicate rock, and fine-grained serendibite occurs with aluminous
diopside
(CM 52:1-14).
Serendibite was discovered in a lens of boron-rich
calc-silicate rock metamorphosed at 6–7 kbar, 650–700oC. The following
stages of mineralisation are recognized:
(1) a prograde assemblage consisting of K-feldspar,
tourmaline and calcite, inferred from relicts in
scapolite
(2A) a peak metamorphic assemblage of aluminous diopside, serendibite, lesser
phlogopite, and local scapolite
(2B) continued formation of phlogopite around serendibite in
calcite pockets although serendibite was stable
(3) high-temperature breakdown of serendibite to uvite +
spinel + calcite, and of aluminous
diopside to pargasite.
The final stage is localised, low-temperature alteration to fine-grained phyllosilicates
(CM 52: 595-615.).
At the Tayezhnoe Fe-B skarn, Aldan, Sakha Republic, Russia, the iron ore deposit contains the
world's most extensive development of serendibite, found
(l) as grains, up to a few centimeters, in magnesian
skarn with clinopyroxene, potassium
pargasite, spinel,
anorthite, uvite,
phlogopite, and, locally, anhydrite
and
(2) as rare grains, 0.02-0.4 mm, in olivine-rich orthosilicate rock with
clinohumite, potassium-rich pargasite,
uvite, spinel,
magnetite, ludwigite and
sinhalite.
The sequence of mineral formation is
(l) formation of skarn containing spinel [1] +
anorthite + clinopyroxene and
plagioclase + clinopyroxene (local
orthopyroxene) during upper
amphibolite to
granulite facies metamorphism (to 850oC, 4-5 kbar)
(2) introduction of boron to form ludwigite,
serendibite and tourmaline (sodian uvite
and uvite) at 600-700oC
(3) replacement of earlier formed serendibite to form uvite,
calcite, spinel [2] and rare
clintonite, and formation of serendibite +
pargasite
(4) low-temperature alteration to chlorite,
clinozoisite, grossular,
margarite and aluminous uvite.
serendibite is a mineral characteristically found in silica-undersaturated skarn
metamorphosed in the presence of H2O-rich fluids at relatively high temperatures (600-825oC) and low to
intermediate pressures (less than l0 kbar)
(AM 76.1061-1080).
At the type locality, Gangapitiya, Ambakotte, Kurunegala District, North Western Province, Sri Lanka,
serendibite was discovered. Bands of granulite, which is composed mainly of
quartz and feldspar, alternate with bands of
limestone up to 18 inches wide. Between
limestone and granulite occur
contact zones consisting, next to the limestone, almost entirely of colourless
diopside, but near the granulite the zones
consist of a mixture of diopside with blue spinel, a
little apatite, occasional scapolite or
plagioclase, and serendipite
(MM 13(6): 224-227)
At Johnsburg Township, Warren county, New York, USA, serendipite is associated with
sinhalite
(Dana).
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