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Category Silicate mineral
(repeating unit)
Crystal symmetry Triclinic H–M Symbol 1
Unit cell a = 7.99 Å, b = 7.03 Å, c = 7.03 Å; α = 90.51°, β = 95.21°, γ = 102.53°; Z = 2
Color Colorless, whitish, grayish, yellowish
Crystal habit Tabular to acicular, radiating fibrous, spheroidal, or columnar; massive
Crystal system Triclinic Pinacoidal
Twinning Twin axis [010] with composition plane [100], common
Cleavage Perfect on {100} and {001}
Fracture Uneven
Tenacity Brittle; tough when compact
Mohs scale hardness 4.5 - 5
Luster Silky, subvitreous
Diaphaneity Translucent to opaque
Specific gravity 2.84 - 2.90
Optical properties Biaxial (+)
Refractive index nα = 1.594 - 1.610 nβ = 1.603 - 1.614 nγ = 1.631 - 1.642
Birefringence δ = 0.037
2V angle Measured: 50° to 63°, Calculated: 42° to 60°
Dispersion r > v weak to very strong
References [1][2][3]

Pectolite is a white to gray mineral, NaCa2Si3O8(OH), sodium calcium inosilicate hydroxide. It crystallizes in the triclinic system typically occurring in radiated or fibrous crystalline masses. It has a Mohs hardness of 4.5 to 5 and a specific gravity of 2.7 to 2.9. The gemstone variety, larimar, is a pale to sky blue.


Spheroidal crystal of Pectolite (from Millington, New Jersey, USA)

It was first described in 1828 at Mt. Baldo, Trento Province, Italy and named from the Greek pektos – "compacted" and lithos – "stone".[2][3]

It occurs as a primary mineral in nepheline syenites, within hydrothermal cavities in basalts and diabase and in serpentinites in association with zeolites, datolite, prehnite, calcite and serpentine. It is found in a wide variety of worldwide locations.

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