How to identify Feldspar with Tourmaline

To correctly identify Feldspar with Tourmaline (Potassium Feldspar (Orthoclase/Microcline) with Schorl (Black Tourmaline)), 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

   Potassium Feldspar: White, pink, salmon-orange, reddish-brown. Schorl: Opaque black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Potassium Feldspar: Vitreous to pearly. Schorl: Vitreous to resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Coarse-grained, often with large, distinct crystals of both minerals. Can exhibit graphic intergrowths.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Potassium Feldspar: Prismatic, tabular, or blocky crystals, often twinned (Carlsbad twinning common). Schorl: Prismatic crystals, typically with a triangular cross-section and vertical striations.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Potassium Feldspar: Two distinct cleavages at or near 90 degrees. Schorl: Poor to indistinct cleavage, often exhibiting conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Granitic pegmatites, often found in the margins of granitic intrusions or within metamorphic terrains.

Key Facts

Hardness

Potassium Feldspar: 6-6.5 on Mohs scale. Schorl: 7-7.5 on Mohs scale.

Specific Gravity

Potassium Feldspar: 2.55-2.63. Schorl: 3.06-3.26.

Crystal System

Potassium Feldspar: Monoclinic (Orthoclase) or Triclinic (Microcline). Schorl: Trigonal.

Color

Potassium Feldspar: White, pink, salmon-orange, reddish-brown. Schorl: Opaque black.

Luster

Potassium Feldspar: Vitreous to pearly. Schorl: Vitreous to resinous.

Transparency

Potassium Feldspar: Translucent to opaque. Schorl: Opaque.

Fracture

Potassium Feldspar: Uneven to conchoidal. Schorl: Conchoidal to uneven.

Cleavage

Potassium Feldspar: Two perfect cleavages at 90 degrees (Orthoclase) or near 90 degrees (Microcline). Schorl: Poor to indistinct.

Composition

Potassium Feldspar: KAlSi3O8. Schorl: Na(Fe2+3)Al6(BO3)3Si6O18(OH)4.

Physical characteristics

• Crystal Habit: Potassium Feldspar: Prismatic, tabular, blocky, often twinned. Schorl: Prismatic, columnar, acicular, often with vertical striations and a characteristic triangular cross-section.
• Cleavage Type: Potassium Feldspar: Perfect in two directions. Schorl: Poor/indistinct.
• Fracture Type: Potassium Feldspar: Uneven to conchoidal. Schorl: Conchoidal to uneven.
• Tenacity: Potassium Feldspar: Brittle. Schorl: Brittle.
• Luster Type: Potassium Feldspar: Vitreous to pearly. Schorl: Vitreous to resinous.

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