How to identify Microcline with Schorl

To correctly identify Microcline with Schorl (KAlSi3O8 (Microcline) and NaFe3Al6(BO3)3Si6O18(OH)4 (Schorl)), 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

   Microcline: White, cream, pale yellow, pink, reddish, or green (amazonite). Schorl: Opaque black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Microcline: Vitreous to pearly. Schorl: Vitreous to resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Coarse-grained, with distinct crystals of both minerals visible. Microcline often shows perthitic exsolution lamellae. Schorl crystals are typically prismatic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microcline: Triclinic, typically forming blocky, anhedral to subhedral crystals, often twinned (e.g., Carlsbad, Baveno, Manebach). Schorl: Hexagonal (trigonal), forming elongated, prismatic crystals with characteristic vertical striations on prism faces and a pseudo-triangular cross-section.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Microcline: Two distinct cleavages at nearly 90 degrees (89°30'). Schorl: Indistinct to poor cleavage, typically exhibiting conchoidal to uneven fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Primarily granitic pegmatites, but also in some granites, aplites, and metamorphic rocks.

Key Facts

Hardness

Microcline: 6-6.5 (Mohs); Schorl: 7-7.5 (Mohs)

Specific Gravity

Microcline: 2.54-2.57; Schorl: 3.03-3.25

Crystal System

Microcline: Triclinic; Schorl: Trigonal (Hexagonal)

Color

Microcline: White, cream, pink, green; Schorl: Opaque black

Luster

Microcline: Vitreous to pearly; Schorl: Vitreous to resinous

Transparency

Microcline: Translucent to opaque; Schorl: Opaque

Fracture

Microcline: Uneven to conchoidal; Schorl: Conchoidal to uneven

Cleavage

Microcline: Perfect in two directions at nearly 90 degrees; Schorl: Indistinct to poor

Composition

Microcline: Potassium aluminum silicate; Schorl: Sodium iron aluminum borosilicate hydroxide

Physical characteristics

• Crystal Habit: Microcline: Blocky, tabular, often twinned; Schorl: Prismatic, columnar, acicular, often striated vertically
• Cleavage Type: Microcline: Perfect {001} and {010} at 89°30'; Schorl: Indistinct {1011} and {0001}
• Fracture Type: Microcline: Uneven to conchoidal; Schorl: Conchoidal to uneven
• Tenacity: Microcline: Brittle; Schorl: Brittle
• Luster Type: Microcline: Vitreous to pearly; Schorl: Vitreous to resinous

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