Stilbite on Basalt

Description

Stilbite on basalt refers to specimens where the zeolite mineral stilbite-Ca has crystallized within the vesicles, vugs, or fractures of a basaltic host rock. Basalt is a fine-grained, dark-colored extrusive igneous rock, rich in iron and magnesium. Stilbite-Ca typically forms radiating, sheaf-like, or tabular crystal aggregates, often displaying a pearly luster, and can range in color from white, colorless, pale yellow, pink, to reddish-brown. The contrast between the delicate, often translucent stilbite crystals and the dark, opaque basalt matrix creates aesthetically pleasing mineral specimens.

How to Identify

Color

Stilbite: White, colorless, pale yellow, pink, salmon, reddish-brown. Basalt: Dark gray to black.

Luster

Stilbite: Vitreous to pearly, especially on cleavage surfaces. Basalt: Dull to sub-vitreous.

Texture

Stilbite: Often forms radiating, sheaf-like aggregates, sometimes tabular crystals. Basalt: Fine-grained, aphanitic, sometimes vesicular or porphyritic.

Crystal Form

Stilbite: Monoclinic, typically forming pseudo-orthorhombic tabular crystals, often twinned, or in characteristic 'sheaf' aggregates. Basalt: Microcrystalline to cryptocrystalline groundmass, with possible phenocrysts of olivine, pyroxene, or plagioclase.

Cleavage

Stilbite: Perfect on {010}. Basalt: No distinct cleavage due to its fine-grained nature.

Geological Environment

Found in the vesicles, amygdules, and fractures of basaltic lava flows, tuffs, and other mafic volcanic rocks. Also occurs in some metamorphic rocks and hydrothermal veins.

Key Facts

• Hardness: Stilbite: 3.5-4 on Mohs scale. Basalt: 6-7 (due to constituent minerals like plagioclase and pyroxene).
• Specific Gravity: Stilbite: 2.12-2.22. Basalt: 2.7-3.3.
• Crystal System: Stilbite: Monoclinic. Basalt: Not applicable (rock, not a single mineral crystal system).
• Color: Stilbite: White, colorless, pale yellow, pink, salmon, reddish-brown. Basalt: Dark gray to black.
• Luster: Stilbite: Vitreous to pearly. Basalt: Dull to sub-vitreous.
• Transparency: Stilbite: Transparent to translucent. Basalt: Opaque.
• Fracture: Stilbite: Uneven to subconchoidal. Basalt: Conchoidal to uneven.
• Cleavage: Stilbite: Perfect on {010}. Basalt: None.
• Composition: Stilbite: Hydrated sodium calcium aluminum silicate (NaCa2Al5Si13O36·14H2O). Basalt: Primarily plagioclase feldspar, pyroxene, and often olivine, with minor amounts of iron-titanium oxides.

Quick Check

• Color: Stilbite: White, pink, salmon. Basalt: Dark gray to black.
• Luster: Stilbite: Pearly to vitreous. Basalt: Dull.
• Streak: Stilbite: White. Basalt: Gray to black.

Physical Characteristics

• Crystal Habit: Stilbite: Tabular, often forming radiating, sheaf-like aggregates, sometimes fibrous or stellate. Basalt: Aphanitic (fine-grained) to porphyritic, vesicular.
• Cleavage Type: Stilbite: Perfect on one direction ({010}). Basalt: Not applicable.
• Fracture Type: Stilbite: Uneven to subconchoidal. Basalt: Conchoidal to uneven.
• Tenacity: Stilbite: Brittle. Basalt: Brittle.
• Luster Type: Stilbite: Vitreous to pearly. Basalt: Dull to sub-vitreous.

Formation

Stilbite-Ca forms as a secondary mineral, typically in the vesicles (gas bubbles) and fractures of basaltic and other mafic volcanic rocks. It crystallizes from low-temperature hydrothermal solutions circulating through the rock after its initial formation. The basalt provides the necessary silica, aluminum, calcium, and other elements, and its porous nature allows for fluid circulation and crystal growth within its cavities.

Usage

Stilbite on basalt is primarily a collector's specimen due to its aesthetic appeal. Stilbite itself, as a zeolite, has industrial applications in ion exchange (water softening), molecular sieves, catalysts, and as an adsorbent. However, specimens on basalt are generally not used for these industrial purposes due to their form and association with the host rock.

Age Distribution

Varies depending on the age of the basaltic host rock, ranging from ancient Precambrian to recent volcanic activity.

Where to Find

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Scientific Classification

Mineral Class

Silicates

Group

Zeolite Group

Crystal System

Monoclinic

Chemical Formula

NaCa2Al5Si13O36·14H2O (Stilbite-Ca end-member)

Composition

Hydrated sodium calcium aluminum silicate

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