Banded Onyx Marble
Calcite (a variety of marble)
Also known as: Onyx Marble, Mexican Onyx, Calcite Onyx, Oriental Alabaster, Travertine Onyx
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Banded Onyx Marble is a translucent to opaque, fine-grained to microcrystalline variety of calcite characterized by distinct, often parallel, bands of varying colors. These bands are typically wavy or concentric, reflecting the depositional environment. Its aesthetic appeal lies in its unique patterns and ability to transmit light, making it popular for backlit applications.
How to Identify
- Color
- Highly variable, often white, cream, yellow, green, brown, red, or a combination of these colors in distinct bands. Impurities dictate the color variations.
- Luster
- Vitreous to waxy, sometimes dull on unpolished surfaces.
- Texture
- Fine-grained to microcrystalline, often compact. Can exhibit a sugary texture on fractured surfaces.
- Crystal Form
- Typically massive, banded, stalactitic, or botryoidal aggregates of microcrystalline calcite. Individual crystals are usually not visible to the naked eye.
- Cleavage
- Perfect rhombohedral cleavage (three directions not at 90 degrees), though often not apparent in massive, fine-grained material.
- Geological Environment
- Forms in karstic environments (caves, sinkholes) as speleothems (stalactites, stalagmites, flowstone) or near hot/cold springs where calcium-rich waters precipitate calcite. It is a chemical sedimentary deposit.
Key Facts
- Hardness: 3 (Mohs scale)
- Specific Gravity: 2.71 g/cm³
- Crystal System: Trigonal
- Color: Highly variable, often banded in white, cream, yellow, green, brown, red
- Luster: Vitreous to waxy
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect rhombohedral (3 directions at 74°55')
- Composition: Calcium Carbonate (CaCO3)
Quick Check
- Color: Banded, various colors (white, cream, yellow, green, brown, red)
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, banded, stalactitic, botryoidal, granular aggregates
- Cleavage Type: Rhombohedral
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to waxy
Formation
Banded Onyx Marble is a form of travertine or speleothem (cave deposit) composed primarily of calcite. It forms from the precipitation of calcium carbonate from cold or hot spring waters, or from groundwater in caves. The banding results from variations in water chemistry, flow rate, and the presence of impurities (e.g., iron oxides for reds/browns, organic matter for grays/blacks) during deposition. It is not a true metamorphic marble, which forms from the recrystallization of limestone under heat and pressure, but rather a chemical sedimentary rock.
Usage
Primarily used as an ornamental stone for interior decoration (countertops, wall cladding, flooring, decorative objects), sculptures, and architectural elements due to its attractive banding and translucency. Historically, it was used for carvings and vessels.
Age Distribution
Ranges from Cenozoic to Mesozoic, depending on the specific deposit, often forming in geologically active areas with hydrothermal or karstic activity.
Where to Find
Mexico
Significant deposits, particularly in Puebla and Oaxaca, known for producing high-quality, colorful banded onyx.
Pakistan
Balochistan province is a major source of green and multi-colored onyx marble.
Iran
Known for various colors, including white, green, and pink onyx marble.
Turkey
Produces a range of colors, often associated with travertine deposits.
Egypt
Historically significant deposits, often referred to as 'Oriental Alabaster'.
USA
Deposits found in Arizona, Utah, and California, often associated with hot springs or cave systems.
Finding Tips
Geological Context
Look for areas with active or ancient hot/cold springs, or karstic limestone terrains with caves. These are the primary environments for its formation.
Surface Features
Examine exposed rock faces for layered or banded structures, often with a waxy or translucent appearance. Weathered surfaces might be duller but still show banding.
Acid Test
Calcite reacts vigorously with dilute hydrochloric acid (HCl), producing effervescence (fizzing). This is a key diagnostic test to distinguish it from true onyx (chalcedony) or gypsum alabaster.
Hardness Test
Calcite has a Mohs hardness of 3, meaning it can be scratched by a copper coin. This helps differentiate it from harder minerals like quartz (Mohs 7).
Similar Rocks
True Marble
Calcite (CaCO3) or Dolomite (CaMg(CO3)2)
Also known as: Crystalline Limestone
Travertine
Calcite (CaCO3)
Also known as: Tufa
Alabaster (Gypsum variety)
Gypsum (CaSO4·2H2O)
Also known as: Gypsum Alabaster
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Calcite Group
- Crystal System
- Trigonal
- Chemical Formula
- CaCO3
- Composition
- Calcium Carbonate
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