Stilbite

Description

Stilbite is a tectosilicate mineral characterized by its distinctive sheaf-like or radiating crystal aggregates, often resembling bundles of wheat. It is a member of the zeolite group, known for its porous structure and ability to exchange ions and absorb water. The most common variety is Stilbite-Ca, which is calcium-rich. Stilbite-Na is a less common sodium-rich variety. It typically forms in the late stages of volcanic rock alteration, filling voids and fissures. Its delicate appearance and often vibrant colors make it a favorite among mineral enthusiasts.

How to Identify

Color

White, colorless, pale yellow, pink, salmon-orange, reddish-brown. Pink to salmon-orange are very characteristic.

Luster

Vitreous (glassy) to pearly on cleavage surfaces.

Texture

Often forms radiating, sheaf-like aggregates or tabular crystals. Can also be fibrous or granular.

Crystal Form

Monoclinic. Crystals are typically tabular, often forming characteristic 'sheaf-of-wheat' aggregates, divergent sprays, or bow-tie shapes. Individual crystals are often elongated along the c-axis.

Cleavage

Perfect on {010}, yielding pearly luster on this surface.

Geological Environment

Amygdaloidal cavities and fractures in basalt and other mafic volcanic rocks; hydrothermal veins; granite pegmatites; some metamorphic rocks.

Key Facts

• Hardness: 3.5-4 on the Mohs scale
• Specific Gravity: 2.12-2.22
• Crystal System: Monoclinic
• Color: White, colorless, pale yellow, pink, salmon-orange, reddish-brown
• Luster: Vitreous to pearly
• Transparency: Transparent to translucent
• Fracture: Uneven
• Cleavage: Perfect on {010}
• Composition: Hydrous calcium sodium aluminum silicate

Quick Check

• Color: White, pale yellow, pink, salmon-orange, reddish-brown
• Luster: Vitreous to pearly
• Streak: White

Physical Characteristics

• Crystal Habit: Typically forms tabular crystals, often aggregated into characteristic 'sheaf-of-wheat' bundles, divergent sprays, or bow-tie shapes. Can also be fibrous, columnar, or granular.
• Cleavage Type: Perfect on one plane ({010}), leading to a pearly luster on this surface.
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy) on crystal faces, pearly on cleavage surfaces.

Formation

Stilbite is a hydrous calcium sodium aluminum silicate mineral belonging to the zeolite group. It typically forms as a secondary mineral in the amygdaloidal cavities and fractures of basaltic and other mafic volcanic rocks. It can also occur in hydrothermal veins, granite pegmatites, and some metamorphic rocks. Its formation is generally associated with low-temperature hydrothermal alteration processes, where circulating fluids interact with primary minerals, leading to the precipitation of zeolites.

Usage

Stilbite has limited industrial uses compared to some other zeolites. It is primarily a collector's mineral due to its attractive crystal forms and colors. Historically, zeolites, including stilbite, have been investigated for their ion-exchange properties, molecular sieving capabilities, and catalytic applications, though other zeolites are more commonly employed for these purposes. Its aesthetic appeal makes it popular in mineral collections and sometimes used in lapidary arts for decorative purposes.

Age Distribution

Stilbite forms in a variety of geological settings and is not restricted to a specific geological age. It is commonly found in Cenozoic and Mesozoic volcanic rocks, but can also occur in older metamorphic and sedimentary rocks.

Where to Find

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Scientific Classification

Mineral Class

Silicates

Group

Zeolite Group (Heulandite-Stilbite Series)

Crystal System

Monoclinic

Chemical Formula

NaCa4(Si27Al9)O72·28H2O (Stilbite-Ca, the most common variety, has a simplified formula of CaAl2Si7O18·7H2O, but the more precise formula reflects the Na substitution and full hydration)

Composition

Hydrous calcium sodium aluminum silicate. The ratio of Si to Al is typically 3:1, and the water content is significant.

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