Marble

Description

Marble is a non-foliated metamorphic rock composed predominantly of recrystallized carbonate minerals, most commonly calcite (CaCO3) or dolomite (CaMg(CO3)2). It is characterized by its interlocking, equigranular crystalline texture. Pure marble is white, but the presence of various mineral impurities (e.g., iron oxides, clay minerals, graphite, serpentine, tremolite) can impart a wide range of colors, including pink, red, yellow, green, black, and gray, often with swirling patterns or veins. Its hardness is relatively low, making it susceptible to acid erosion.

How to Identify

Color

Typically white in its purest form, but can be pink, red, yellow, green, black, gray, or variegated due to impurities. Veining is common.

Luster

Vitreous to dull, depending on the grain size and presence of impurities. Polished surfaces exhibit a high vitreous luster.

Texture

Granoblastic (equigranular, interlocking crystals), non-foliated. Grain size can range from fine to coarse.

Crystal Form

Individual crystals are typically anhedral (lacking well-formed faces) due to mutual interference during growth, forming a mosaic.

Cleavage

Calcite and dolomite, the primary minerals, exhibit perfect rhombohedral cleavage in three directions, which can sometimes be observed in larger grains within the marble.

Geological Environment

Formed in regional metamorphic belts associated with convergent plate boundaries (orogenic belts) or by contact metamorphism adjacent to igneous intrusions. Occurs in areas where thick sequences of limestone or dolostone have been subjected to elevated temperatures and pressures.

Key Facts

• Hardness: 3-4 on Mohs scale (for calcite-rich marble); 3.5-4 on Mohs scale (for dolomite-rich marble).
• Specific Gravity: 2.7-2.9 (for calcite marble); 2.8-2.9 (for dolomitic marble).
• Crystal System: Trigonal (for calcite and dolomite).
• Color: Variable, from pure white to various hues and patterns.
• Luster: Vitreous to dull.
• Transparency: Opaque to translucent, rarely transparent in very pure, coarse-grained varieties.
• Fracture: Uneven to subconchoidal.
• Cleavage: Perfect rhombohedral in three directions (characteristic of calcite and dolomite).
• Composition: Primarily calcite (CaCO3) or dolomite (CaMg(CO3)2), with various accessory minerals.

Quick Check

• Color: White, pink, red, yellow, green, black, gray, or variegated.
• Luster: Vitreous to dull (unpolished), high vitreous (polished).
• Streak: White.

Physical Characteristics

• Crystal Habit: Granular, massive, interlocking anhedral crystals.
• Cleavage Type: Perfect rhombohedral (3 directions at 74° and 106°).
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to dull.

Formation

Marble forms from the metamorphism of carbonate sedimentary rocks, primarily limestone or dolostone. During regional or contact metamorphism, the original carbonate minerals (calcite, CaCO3, or dolomite, CaMg(CO3)2) recrystallize. This process obliterates the original sedimentary textures and structures, such as bedding planes and fossils, and results in a mosaic of interlocking, equigranular carbonate crystals. The degree of recrystallization depends on temperature, pressure, and the duration of metamorphism. Impurities in the original carbonate rock, such as clay minerals, quartz, iron oxides, or organic matter, can react during metamorphism to form new silicate minerals (e.g., tremolite, diopside, forsterite, wollastonite, garnet, mica) or graphite, which contribute to the varied colors and patterns observed in marble.

Usage

Marble has been used for millennia in sculpture, architecture (flooring, wall cladding, countertops, columns), and monuments due to its aesthetic appeal, relative softness for carving, and ability to take a high polish. Industrial uses include crushed stone for construction aggregate, dimension stone, and as a source of calcium carbonate for various applications (e.g., agricultural lime, fillers in paint and paper, neutralizing agent for acids).

Age Distribution

Ranges from Precambrian to Cenozoic, depending on the age of the protolith (limestone or dolostone) and the timing of metamorphism.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Carbonate rock (metamorphic)

Group

Metamorphic rocks

Crystal System

Trigonal (for constituent minerals calcite/dolomite)

Chemical Formula

CaCO3 (calcite marble) or CaMg(CO3)2 (dolomitic marble)

Composition

Predominantly calcium carbonate or calcium magnesium carbonate, with potential impurities like quartz, mica, graphite, iron oxides, and various silicates.

Explore Marble