Analcime - Na(Si₂Al)O₆·H₂O

Habit: White, colorless, gray, pink or greenish yellow. Crystals commonly trapezohedra; also granular, compact or massive, typically showing concentric structure. Vitreous luster; transparent to translucent. White streak.

Environment: Occurs in the groundmass or vesicles of silica-poor intermediate and mafic igneous rocks, typically basalts and phonolites, originating from late-stage hydrothermal solutions or disseminated because of alteration. Also found in lakebeds, altered from pyroclastics or clays, or as a primary precipitate, and authigenic in sandstones and siltstones.

Etymology: From the Greek word for "weak," alluding to the weak electrostatic charge analcime develops when heated or rubbed.