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Stilbite on basalt refers to specimens where the zeolite mineral Stilbite-Ca (calcium-rich stilbite) has crystallized within the vesicles or on the surfaces of fractures within a basaltic rock matrix. Basalt is a common extrusive igneous rock, typically dark gray to black, fine-grained, and often vesicular. Stilbite-Ca commonly forms distinctive 'sheaf-like' or 'bow-tie' aggregates of tabular crystals, often radiating outwards. The color of stilbite-Ca can vary from white, colorless, and pale yellow to salmon pink, orange, or reddish-brown, providing a striking contrast against the dark basalt.
How to Identify
- Color
- Stilbite-Ca: White, colorless, pale yellow, salmon pink, orange, reddish-brown. Basalt: Dark gray to black. The contrast is key.
- Luster
- Stilbite-Ca: Vitreous to pearly on cleavage surfaces. Basalt: Dull to sub-vitreous.
- Texture
- Stilbite-Ca: Often forms radiating, sheaf-like, or bow-tie aggregates of tabular crystals. Basalt: Fine-grained, sometimes vesicular (containing small holes from gas bubbles).
- Crystal Form
- Stilbite-Ca: Monoclinic, typically forming tabular crystals often twinned to create distinctive 'sheaf' or 'bow-tie' aggregates. Basalt: Aphanitic (fine-grained) to porphyritic (larger crystals in a fine matrix).
- Cleavage
- Stilbite-Ca: Perfect on {010}. Basalt: No distinct cleavage, but can exhibit fracture.
- Geological Environment
- Found in the vesicles and fractures of basaltic lava flows, often associated with other zeolites and secondary minerals.
Key Facts
- Hardness: Stilbite-Ca: 3.5-4 on the Mohs scale. Basalt: 6-7 (due to constituent minerals like plagioclase and pyroxene).
- Specific Gravity: Stilbite-Ca: 2.12-2.22 g/cm³. Basalt: 2.8-3.0 g/cm³.
- Crystal System: Stilbite-Ca: Monoclinic. Basalt: Not applicable (rock, not a single crystal system).
- Color: Stilbite-Ca: White, colorless, pale yellow, salmon pink, orange, reddish-brown. Basalt: Dark gray to black.
- Luster: Stilbite-Ca: Vitreous to pearly. Basalt: Dull to sub-vitreous.
- Transparency: Stilbite-Ca: Transparent to translucent. Basalt: Opaque.
- Fracture: Stilbite-Ca: Uneven to subconchoidal. Basalt: Conchoidal to uneven.
- Cleavage: Stilbite-Ca: Perfect on {010}. Basalt: No distinct cleavage.
- Composition: Stilbite-Ca: Hydrated calcium sodium aluminum silicate, (Ca,Na)9[Al9Si27O72]·nH2O. Basalt: Primarily plagioclase feldspar, pyroxene, and often olivine, with minor amounts of iron oxides.
Quick Check
- Color: Stilbite-Ca: White, pink, orange, brown. Basalt: Dark gray to black.
- Luster: Stilbite-Ca: Vitreous to pearly. Basalt: Dull to sub-vitreous.
- Streak: Stilbite-Ca: White. Basalt: Gray to black.
Physical Characteristics
- Crystal Habit: Stilbite-Ca: Typically forms radiating, sheaf-like, or 'bow-tie' aggregates of tabular crystals, often twinned. Individual crystals are monoclinic.
- Cleavage Type: Stilbite-Ca: Perfect on {010}, yielding pearly luster on cleavage surfaces. Basalt: Not applicable (rock).
- Fracture Type: Stilbite-Ca: Uneven to subconchoidal. Basalt: Conchoidal to uneven.
- Tenacity: Stilbite-Ca: Brittle. Basalt: Brittle.
- Luster Type: Stilbite-Ca: Vitreous to pearly. Basalt: Dull to sub-vitreous.
Formation
Stilbite-Ca forms as a secondary mineral in the vesicles (gas bubbles) and fractures of basaltic lavas. After the basalt cools and solidifies, hydrothermal fluids, rich in dissolved silica, aluminum, and alkali/alkaline earth elements, circulate through the rock. These fluids react with the basalt and precipitate zeolites, including stilbite-Ca, in open spaces. The characteristic 'sheaf-like' or 'bow-tie' crystal aggregates of stilbite-Ca often grow directly on the dark, fine-grained basalt matrix.
Usage
Stilbite on basalt specimens are primarily valued by mineral collectors for their aesthetic appeal and the contrast between the delicate, often colorful stilbite crystals and the dark basalt matrix. Stilbite-Ca itself has some minor industrial applications as a molecular sieve, catalyst, and ion-exchange material, though other zeolites are more commonly used.
Age Distribution
Stilbite-Ca typically forms as a secondary mineral in basaltic rocks, which can range in age from recent to hundreds of millions of years old, depending on the specific volcanic activity.
Where to Find
Deccan Traps, India
World-renowned for exceptional zeolite specimens, including stilbite-Ca on basalt, particularly from quarries near Nashik, Pune, and Mumbai.
Iceland
Numerous localities, especially in eastern Iceland, yield excellent stilbite-Ca specimens in basaltic environments.
Faroe Islands
Known for high-quality zeolite occurrences, including stilbite-Ca, in basaltic rocks.
Nova Scotia, Canada
Coastal basalt cliffs, particularly around the Bay of Fundy, are famous for zeolite minerals, including stilbite-Ca.
Oregon and Washington, USA
Basalt flows in the Pacific Northwest, such as the Columbia River Basalt Group, contain various zeolites, including stilbite-Ca.
Finding Tips
Look for Vesicles and Fractures
Stilbite-Ca typically forms in the open spaces within basalt. Examine exposed basalt outcrops for vugs (cavities) or veins where crystals might have grown.
Identify Associated Minerals
Stilbite-Ca often co-occurs with other zeolites (e.g., heulandite, scolecite, analcime) and secondary minerals (e.g., calcite, apophyllite). Learning to identify these can help locate stilbite.
Check Basaltic Lava Flows
Focus your search on areas with extensive basaltic lava flows, especially those that have undergone low-temperature hydrothermal alteration.
Safety Precautions
When collecting in basaltic terrains, be aware of unstable rock faces, sharp edges, and potential for falling rocks. Always wear appropriate safety gear, including hard hats, gloves, and eye protection. Some basaltic areas may contain naturally occurring asbestos minerals (e.g., chrysotile, tremolite, actinolite) or other fibrous silicates, so exercise caution and avoid disturbing or inhaling dust from such materials. Always wash hands after handling specimens.
Similar Rocks
Heulandite on Basalt
Heulandite-Ca on Basalt
Also known as: Heulandite-Ca on Basalt
Apophyllite on Basalt
Fluorapophyllite-K on Basalt
Also known as: Fluorapophyllite-K on Basalt
Scolecite on Basalt
Scolecite on Basalt
Also known as: Scolecite on Basalt
Scientific Classification
- Mineral Class
- Silicates
- Group
- Zeolite Group (specifically, the Heulandite-Stilbite Series)
- Crystal System
- Monoclinic
- Chemical Formula
- (Ca,Na)9[Al9Si27O72]·nH2O (where n is typically around 28-30)
- Composition
- Hydrated calcium sodium aluminum silicate. The 'Ca' in Stilbite-Ca indicates that calcium is the dominant extra-framework cation, distinguishing it from Stilbite-Na.
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