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Onyx Marble is a translucent to opaque, banded variety of calcite. It is characterized by its parallel bands of varying colors, which can include white, cream, yellow, green, brown, and red, often in swirling or wavy patterns. The banding is a result of rhythmic deposition of calcium carbonate and impurities. Despite its common name, it is not true onyx (which is a cryptocrystalline quartz) nor true marble (which is metamorphosed limestone), but rather a form of travertine or speleothem composed predominantly of calcite.
How to Identify
- Color
- Highly variable, often white, cream, yellow, green, brown, red, or a combination, typically in distinct bands.
- Luster
- Vitreous to waxy when polished, dull to earthy on unpolished surfaces.
- Texture
- Fine-grained to medium-grained, often with a sugary or granular feel on broken surfaces. Smooth and cool to the touch when polished.
- Crystal Form
- Typically massive, microcrystalline to cryptocrystalline, forming banded layers. Individual crystals are usually not visible to the naked eye.
- Cleavage
- Perfect rhombohedral cleavage (three directions not at 90 degrees), though rarely observed in massive, fine-grained specimens.
- Geological Environment
- Found in caves (as stalactites, stalagmites, flowstone), around hot or cold springs (as travertine), and in other environments where calcium-rich waters precipitate calcite.
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, sometimes dull
- 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 (polished), dull (unpolished)
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, microcrystalline to cryptocrystalline, forming banded layers (speleothems, travertine). Individual crystals are typically microscopic.
- Cleavage Type: Perfect rhombohedral, {1011}
- Fracture Type: Conchoidal to uneven, sometimes splintery
- Tenacity: Brittle
- Luster Type: Vitreous to waxy on fresh or polished surfaces; dull to earthy on unpolished, weathered surfaces.
Formation
Onyx Marble is a form of banded calcite, primarily formed by the precipitation of calcium carbonate (CaCO3) from cold or hot aqueous solutions. This typically occurs in caves as stalagmites and stalactites (speleothems), or in spring deposits (travertine). The banding results from variations in water flow, mineral content, and organic impurities during deposition. While often referred to as 'marble' due to its appearance and polishability, it is not a true metamorphic marble (which is recrystallized limestone) but rather a chemical sedimentary rock. However, some deposits may undergo low-grade metamorphism, leading to slight recrystallization.
Usage
Primarily used as an ornamental and decorative stone for interior applications such as countertops, wall cladding, flooring, sculptures, and decorative objects. Its translucency makes it popular for backlit installations. Historically, it has been used for carvings and architectural elements.
Age Distribution
Can form in various geological periods, from Precambrian to Cenozoic, wherever suitable conditions for calcite precipitation exist.
Where to Find
Mexico
Renowned for high-quality, often green and white banded Onyx Marble, particularly from Puebla and Oaxaca.
Pakistan
Known for producing a wide range of colors, including green, white, and multi-colored varieties.
Iran
Historically significant source, often referred to as 'Oriental Alabaster', with beautiful translucent varieties.
Turkey
Produces various colors, including white, yellow, and brown banded types.
Egypt
Ancient source, particularly for yellowish and reddish varieties used in historical artifacts.
USA
Deposits found in various states, often associated with cave systems (e.g., New Mexico, Arizona) and hot springs.
Finding Tips
Geological Context
Look for areas with limestone bedrock, active or ancient cave systems, or hot/cold spring deposits. These are the primary environments for calcite precipitation.
Surface Features
Examine exposed rock faces for banded patterns, often associated with flowstone or layered deposits. Weathered surfaces may appear dull, but a fresh break or wet surface will reveal the characteristic banding and translucency.
Acid Test
Since Onyx Marble is primarily calcite, it will effervesce (fizz) vigorously when a drop of dilute hydrochloric acid (HCl) is applied. This is a key diagnostic test to distinguish it from true onyx (quartz) or gypsum alabaster.
Hardness Test
Calcite has a Mohs hardness of 3. It can be scratched by a copper coin (hardness 3.5) but will scratch a fingernail (hardness 2.5). This helps differentiate it from harder true onyx (hardness 7) and softer gypsum alabaster (hardness 2).
Similar Rocks
True Onyx
Chalcedony (SiO2)
Also known as: Black Onyx, Sardonyx
Travertine
Calcite (CaCO3)
Also known as: Tufa
Marble
Recrystallized Calcite (CaCO3) or Dolomite (CaMg(CO3)2)
Also known as: Metamorphic Limestone
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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