Corundum in Feldspathic Matrix

Description

Corundum in a feldspathic matrix refers to a rock where crystals of the mineral corundum (aluminum oxide, Al2O3) are embedded within a groundmass predominantly composed of feldspar minerals. The corundum crystals can vary significantly in size, from microscopic grains to large, well-formed hexagonal prisms or barrel-shaped crystals. Their color can range from colorless, white, gray, blue (sapphire), red (ruby), pink, yellow, or brown, depending on trace element impurities. The feldspathic matrix typically appears white, gray, pink, or greenish, reflecting the color of the dominant feldspar (e.g., plagioclase, orthoclase, microcline). The texture of the matrix can be fine-grained to coarse-grained, and the rock may exhibit a massive, foliated, or granular structure depending on its metamorphic or igneous history. The contrast in hardness between the very hard corundum and the softer feldspar (Mohs 6-6.5) is a key characteristic.

How to Identify

Color

Corundum crystals can be various colors (red, blue, pink, white, gray, brown, colorless). The feldspathic matrix is typically white, gray, pink, or greenish. The overall rock color will be a combination of these.

Luster

Corundum exhibits a vitreous to adamantine luster. Feldspar has a vitreous to pearly luster.

Texture

The texture can be granular, massive, or foliated. Corundum crystals are often anhedral to euhedral, embedded within a finer-grained to coarse-grained feldspathic groundmass.

Crystal Form

Corundum commonly forms hexagonal prisms, barrel-shaped crystals, or tabular crystals. Feldspar crystals are typically blocky or tabular.

Cleavage

Corundum has no true cleavage but exhibits parting on {0001} and {1011}. Feldspar has two distinct cleavages at or near 90 degrees.

Geological Environment

Typically found in metamorphic terrains (e.g., amphibolite to granulite facies rocks, desilicated pegmatites, contact metamorphic aureoles) or in certain igneous rocks (e.g., anorthosites, nepheline syenites, or desilicated mafic/ultramafic intrusions).

Key Facts

• Hardness: Corundum: 9 (Mohs); Feldspar: 6-6.5 (Mohs)
• Specific Gravity: Corundum: 3.95-4.10; Feldspar: 2.55-2.76
• Crystal System: Corundum: Trigonal; Feldspar: Monoclinic or Triclinic
• Color: Corundum: Red, blue, pink, white, gray, brown, colorless; Feldspar: White, gray, pink, green
• Luster: Vitreous to adamantine (corundum); Vitreous to pearly (feldspar)
• Transparency: Corundum: Transparent to opaque; Feldspar: Transparent to translucent
• Fracture: Corundum: Uneven to conchoidal; Feldspar: Uneven to conchoidal
• Cleavage: Corundum: No true cleavage, but parting on {0001} and {1011}; Feldspar: Two distinct cleavages at or near 90 degrees
• Composition: Corundum: Aluminum oxide (Al2O3); Feldspar: Aluminosilicate of potassium, sodium, or calcium (e.g., KAlSi3O8, NaAlSi3O8, CaAl2Si2O8)

Quick Check

• Color: Variable (corundum: red, blue, white, gray; matrix: white, gray, pink)
• Luster: Vitreous to adamantine (corundum); Vitreous to pearly (feldspar)
• Streak: White (for both corundum and feldspar)

Physical Characteristics

• Crystal Habit: Corundum: Hexagonal prisms, barrel-shaped, tabular, massive, granular; Feldspar: Blocky, tabular, massive, granular
• Cleavage Type: Corundum: Parting (not true cleavage); Feldspar: Perfect to good in two directions
• Fracture Type: Uneven to conchoidal for both
• Tenacity: Brittle for both
• Luster Type: Vitreous to adamantine (corundum); Vitreous to pearly (feldspar)

Formation

Corundum in a feldspathic matrix typically forms under specific geological conditions involving high aluminum content and often high temperatures and pressures. It can originate from several processes: 1. Metamorphism of aluminum-rich sedimentary rocks (e.g., bauxites, shales) or igneous rocks (e.g., anorthosites, gabbros) under amphibolite to granulite facies conditions. The breakdown of aluminum silicates (like kyanite, sillimanite, or andalusite) can lead to corundum formation. 2. Desilication of aluminous magmas or pegmatites intruding into ultramafic rocks (e.g., serpentinites). The reaction between the silica-undersaturated magma and the silica-poor host rock can lead to the crystallization of corundum. 3. Contact metamorphism of aluminous rocks adjacent to igneous intrusions. 4. Hydrothermal alteration processes, though less common for significant corundum deposits in feldspathic matrices. The feldspathic matrix (composed primarily of feldspar minerals like plagioclase or K-feldspar) provides the bulk of the rock and often indicates the original protolith or the metamorphic conditions. The presence of corundum signifies aluminum saturation and silica undersaturation or depletion during its formation.

Usage

Corundum itself is highly valued for its hardness (Mohs 9) and is used as an abrasive, in industrial bearings, and as a gemstone (ruby and sapphire). Corundum in matrix, especially if the corundum crystals are of gem quality or significant size, can be collected as a mineral specimen. If the corundum content is high enough, the rock might be processed for industrial corundum. The matrix itself, being feldspathic, generally has no specific industrial use beyond its role as a host rock, though some feldspar-rich rocks are used in ceramics or glassmaking.

Age Distribution

Varies widely, from Precambrian to Cenozoic, depending on the specific geological setting and host rock formation.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Oxides (Corundum); Tectosilicates (Feldspar)

Group

Corundum Group (Corundum); Feldspar Group (Feldspar)

Crystal System

Trigonal (Corundum); Monoclinic or Triclinic (Feldspar)

Chemical Formula

Al2O3 (Corundum); KAlSi3O8, NaAlSi3O8, CaAl2Si2O8 (Feldspar)

Composition

Corundum is pure aluminum oxide. Feldspars are framework silicates of aluminum, silicon, and oxygen with potassium, sodium, or calcium.

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