Pectolite

(repeating unit)NaCa₂Si₃O₈(OH)IMA symbolPct^[1]Crystal systemTriclinicCrystal classPinacoidal (1)
(same H-M symbol)Space groupP1Unit cella = 7.99 Å, b = 7.03 Å,
c = 7.03 Å; α = 90.51°,
β = 95.21°, γ = 102.53°; Z = 2IdentificationColorColorless, whitish, grayish, yellowishCrystal habitTabular to acicular, radiating fibrous, spheroidal, or columnar; massiveTwinningTwin axis [010] with composition plane [100], commonCleavagePerfect on {100} and {001}FractureUnevenTenacityBrittle; tough when compactMohs scale hardness4.5 – 5LusterSilky, subvitreousStreakWhiteDiaphaneityTranslucent to opaqueSpecific gravity2.84 – 2.90Optical propertiesBiaxial (+)Refractive indexn_α = 1.594 – 1.610 n_β = 1.603 – 1.614 n_γ = 1.631 – 1.642Birefringenceδ = 0.0372V angleMeasured: 50° to 63°, Calculated: 42° to 60°Dispersionr > v weak to very strongReferences^[2][3][4]

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. There is also a whitish form of the mineral from Alaska that is sometimes marketed as 'Alaska jade'.

It was first described in 1828 at Mount Baldo, Trento Province, Italy, and named from the Greek pektos – "compacted" and lithos – "stone".^[3][4]

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.

• Serandite – the manganese analogue

• Mineral galleries