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Mesolite
Mesolite from Bombay collected in the 18th century by Dr John Hunter
General
Category	Tectosilicate minerals
Group	Zeolite group, natrolite subgroup
Formula	Na2Ca2Si9Al6O30·8H2O
IMA symbol	Mes
Strunz classification	9.GA.05
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-M symbol)
Space group	Fdd2
Unit cell	a = 18.4049(8) Å, b = 56.655(6) Å, c = 6.5443(4) Å; Z = 8
Identification
Color	Colorless, white, gray, yellowish brown
Crystal habit	As elongated prismatic crystals, commonly in hairlike tufts and aggregates of fibers; radiating compact masses; stalactitic; porcelaneous
Twinning	Characteristically twinned on {010} or {100}
Cleavage	Perfect on {110} and {110}
Fracture	Uneven
Tenacity	Brittle, masses tough
Mohs scale hardness	5
Luster	Vitreous, silky when fibrous
Streak	White
Diaphaneity	Transparent to translucent, opaque
Specific gravity	2.26
Optical properties	Biaxial (+)
Refractive index	nα = 1.505 nβ = 1.505 nγ = 1.505
Birefringence	δ = 0.001
2V angle	Measured: 80°
Other characteristics	May exhibit a small pyroelectric effect; piezoelectric
References

Mesolite is a tectosilicate mineral with formula Na₂Ca₂Si₉Al₆O₃₀·8H₂O. It is a member of the zeolite group and the natrolite subgroup, also resembling natrolite in appearance.

Mesolite crystallizes in the orthorhombic system and typically forms fibrous, acicular prismatic crystals or masses. Radiating sprays of needlelike crystals are not uncommon. It is vitreous in luster and clear to white in color. It has a Mohs hardness of 5 to 5.5 and a low specific gravity of 2.2 to 2.4. The refractive indices are nα=1.505 nβ=1.505 nγ=1.506.

Occurrence

It was first described in 1816 for an occurrence in the Cyclopean Islands near Catania, Sicily. From the Greek mesos, "middle", as its composition lies between natrolite and scolecite. Like other zeolites, mesolite occurs as void fillings in amygdaloidal basalt also in andesites and hydrothermal veins.

Images

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  Mesolite
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  "Puff ball" of mesolite in a basaltic vug
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  Mesolite often forms in fibrous crystals
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  A pocket of hairlike acicular crystals of mesolite growing off thomsonite
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  Fibrous crystal sample retrieved from caverns near Pune, India

References

• Mineral Galleries