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Category Inosilicate mineral
(repeating unit)
Crystal system Triclinic
Crystal class Pinacoidal (1)
(same H-M symbol)
Space group P1
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
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 hydroxide inosilicate. 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.


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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