How to identify Granite with Mafic Enclave

To correctly identify Granite with Mafic Enclave (Granite (felsic igneous rock) with a darker, finer-grained mafic enclave (igneous inclusion)), 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

   Host granite: typically light-colored (pink, red, gray, white) due to high feldspar and quartz content. Mafic enclave: distinctly darker (dark gray, black, dark green) due to higher content of mafic minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Host granite: Vitreous to pearly for feldspars and quartz, sub-vitreous to pearly for micas. Mafic enclave: Similar, but overall appearance may be duller due to finer grain size and higher mafic mineral content.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Host granite: Phaneritic (coarse-grained), equigranular to porphyritic. Mafic enclave: Finer-grained (microgranular), often equigranular, sometimes with porphyritic textures (e.g., plagioclase phenocrysts). The contrast in grain size is a key identifier.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Host granite: Anhedral to subhedral crystals of quartz, feldspar, and micas. Mafic enclave: Typically subhedral to euhedral crystals of plagioclase, hornblende, and biotite, often smaller than in the host.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Host granite: Feldspars exhibit two good cleavages at or near 90 degrees. Micas have perfect basal cleavage. Quartz has no cleavage. Mafic enclave: Plagioclase shows two good cleavages. Hornblende has two cleavages at 56 and 124 degrees. Biotite has perfect basal cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive igneous environments, specifically within granitic batholiths, plutons, and dikes. Often found in convergent plate boundaries where continental crust is thickened and melted, or in anorogenic settings.

Key Facts

Hardness

6-7 (Mohs scale, due to quartz and feldspar)

Specific Gravity

2.6-2.8 g/cm³ (average for granite, enclaves may be slightly higher)

Crystal System

Not applicable for a rock; constituent minerals are mostly monoclinic (feldspars, micas, amphiboles) or trigonal (quartz)

Color

Bimodal: light-colored host, dark-colored inclusions

Luster

Vitreous to pearly

Transparency

Opaque (in hand specimen)

Fracture

Conchoidal (quartz), uneven to splintery (feldspars, micas)

Cleavage

Good to perfect in feldspars and micas; poor to absent in quartz

Composition

Host: Quartz, alkali feldspar, plagioclase, micas/amphiboles. Enclave: Plagioclase, hornblende, biotite, pyroxene, minor quartz/alkali feldspar.

Physical characteristics

• Crystal Habit: Granular, anhedral to subhedral crystals in both host and enclave, with finer grains in the enclave.
• Cleavage Type: Perfect basal (micas), good prismatic (feldspars, amphiboles)
• Fracture Type: Conchoidal (quartz), uneven (feldspars), splintery (micas)
• Tenacity: Brittle
• Luster Type: Vitreous to pearly

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