Pectolite
General
Category	Inosilicate mineral
Formula (repeating unit)	NaCa2Si3O8(OH)
IMA symbol	Pct[1]
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
Identification
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
Streak	White
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	[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.

Occurrence

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.