How to identify Stilbite on Basalt

To correctly identify Stilbite on Basalt (Stilbite-Ca (NaCa2Al5Si13O36·14H2O) on Basalt), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

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

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

Explore Stilbite on Basalt