How to identify Pegmatite

To correctly identify Pegmatite (Pegmatite), 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

   Highly variable, depending on mineralogy. Granitic pegmatites are typically light-colored (white, pink, grey) due to abundant quartz and feldspar, but can contain dark minerals like tourmaline or biotite.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly, depending on the constituent minerals (e.g., quartz is vitreous, feldspar can be vitreous to pearly, mica is pearly).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic, exceptionally coarse-grained. Crystals are typically >1 cm, often several centimeters to meters in size. Graphic texture (intergrowth of quartz and feldspar resembling ancient writing) is common.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals are often euhedral to subhedral, well-formed due to unconstrained growth in a fluid-rich environment. Large, distinct crystals of quartz, feldspar, mica, and various accessory minerals are characteristic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Varies by mineral. Feldspars exhibit two good cleavages at or near 90 degrees. Micas have perfect basal cleavage. Quartz has no cleavage but conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found as dikes, sills, or irregular masses intruding into country rock, often associated with the margins of larger granitic intrusions. They are common in ancient cratonic areas and orogenic belts.

Key Facts

Hardness

Variable, depending on constituent minerals (e.g., quartz 7, feldspar 6-6.5, mica 2-4).

Specific Gravity

Variable, typically 2.6-2.9 for granitic pegmatites, but can be higher with denser accessory minerals.

Crystal System

Not applicable to the rock as a whole; individual minerals have their own crystal systems (e.g., quartz - trigonal, feldspar - monoclinic/triclinic).

Color

Highly variable, commonly light-colored (white, pink, grey) but can include dark minerals.

Luster

Vitreous to pearly.

Transparency

Opaque to translucent for the rock; individual minerals can be transparent to opaque.

Fracture

Irregular to conchoidal (for quartz).

Cleavage

Present in feldspars and micas, absent in quartz.

Composition

Predominantly quartz, feldspar (orthoclase, microcline, albite), and mica (muscovite, biotite). May contain a wide array of accessory minerals, including tourmaline, beryl, spodumene, garnet, topaz, columbite-tantalite, cassiterite, apatite, and rare earth minerals.

Physical characteristics

• Crystal Habit: Individual crystals are often euhedral to subhedral, forming large, distinct grains. Graphic intergrowths are common.
• Cleavage Type: Varies by mineral: good in feldspars (two directions), perfect basal in micas, absent in quartz.
• Fracture Type: Irregular to conchoidal (quartz).
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

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