Stevensite

stevensite

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Formula: (Ca,Na)_xMg_3-ySi₄O₁₀(OH)₂
Phyllosilicate (sheet silicate), smectite group
Crystal System: Monoclinic
Specific gravity: 2.15 to 2.57 measured, 2.07 calculated
Hardness: 2½
Streak: White
Colour: White, pale yellow, pale brown, pale pink
Luminescence: Fluoresces light brown under long wave UV
Common impurities: Ti,Al,Fe,Mn,Na,K,H₂O
Environments

Hydrothermal environments

Stevensite occurs in cavities in basalt, formed by late-stage hydrothermal replacement of pectolite; it is a hydrothermal alteration product of igneous rocks, such as dunite and tuff. It may form authigenically (in place), interstratified with other species. Associated minerals include quartz, talc and pectolite (HOM).

Localities

At the type locality, Bergen Hill, Bergen County & Hudson County, New Jersey, USA, stevensite forms arrow-headed masses in veins in trap rock, associated with calcite (Mindat).

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