How to identify Feldspar with Tourmaline

To correctly identify Feldspar with Tourmaline (Feldspar (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8) with Tourmaline (Na(Mg,Fe,Li,Mn,Al)3Al6(BO3)3Si6O18(OH)4)), 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

   Feldspar: White, pink, grey, cream. Tourmaline: Black (schorl is most common in this association), green, pink, blue, brown, or colorless. The contrast between the light-colored feldspar and often dark tourmaline is a key identifier.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Feldspar: Vitreous to pearly. Tourmaline: Vitreous to resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically crystalline, ranging from fine-grained to very coarse-grained, especially in pegmatites where large crystals of both minerals can be observed. Tourmaline often forms prismatic crystals embedded within or alongside feldspar.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Feldspar: Tabular, blocky, or anhedral grains. Tourmaline: Elongated prismatic crystals, often with characteristic triangular cross-sections and striations parallel to the c-axis.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Granitic pegmatites, aplites, hydrothermal veins, and some high-grade metamorphic rocks (e.g., schists, gneisses) where boron-rich fluids were present.

Key Facts

Hardness

Feldspar: 6-6.5 (Mohs). Tourmaline: 7-7.5 (Mohs).

Specific Gravity

Feldspar: 2.55-2.76. Tourmaline: 3.0-3.26.

Crystal System

Feldspar: Monoclinic (orthoclase, sanidine) or Triclinic (plagioclase, microcline). Tourmaline: Trigonal.

Color

Feldspar: White, pink, grey, cream. Tourmaline: Black, green, pink, blue, brown, colorless.

Luster

Vitreous to pearly (feldspar), Vitreous to resinous (tourmaline).

Transparency

Feldspar: Translucent to opaque. Tourmaline: Transparent to opaque.

Fracture

Feldspar: Uneven to conchoidal. Tourmaline: Conchoidal to uneven.

Cleavage

Feldspar: Two distinct cleavages at or near 90 degrees. Tourmaline: Poor to indistinct.

Composition

Feldspar: Aluminosilicates of potassium, sodium, and calcium. Tourmaline: Complex borosilicate with varying amounts of Na, Mg, Fe, Li, Mn, Al.

Physical characteristics

• Crystal Habit: Feldspar: Tabular, blocky, anhedral. Tourmaline: Prismatic, acicular, columnar, often with striations parallel to the c-axis and a characteristic triangular cross-section.
• Cleavage Type: Feldspar: Perfect in two directions. Tourmaline: Poor to indistinct.
• Fracture Type: Feldspar: Uneven to conchoidal. Tourmaline: Conchoidal to uneven.
• Tenacity: Feldspar: Brittle. Tourmaline: Brittle.
• Luster Type: Feldspar: Vitreous to pearly. Tourmaline: Vitreous to resinous.

Explore Feldspar with Tourmaline