How to identify Corundum in Feldspathic Matrix

To correctly identify Corundum in Feldspathic Matrix (Corundum (Al2O3) in a feldspathic matrix), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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)

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