How to identify Granite with Mafic Dike

To correctly identify Granite with Mafic Dike (Granite (felsic intrusive igneous rock) with mafic dike (igneous intrusion)), 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

   Granite: Typically light-colored (white, pink, gray, or red) with dark specks. Mafic Dike: Dark gray to black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Granite: Vitreous to dull for individual mineral grains. Mafic Dike: Dull to sub-vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Granite: Phaneritic (coarse-grained), interlocking crystals, typically equigranular. Mafic Dike: Aphanitic (fine-grained) to medium-grained (e.g., ophitic in diabase), often with chilled margins near the granite contact.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Granite: Anhedral to subhedral crystals of quartz, feldspar, and mica. Mafic Dike: Typically anhedral, small crystals, sometimes porphyritic with phenocrysts of plagioclase or pyroxene.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Granite: Feldspars exhibit good cleavage (two directions at ~90 degrees), micas have perfect basal cleavage. Quartz has no cleavage. Mafic Dike: Pyroxenes have two cleavages at ~90 degrees, amphiboles at ~60/120 degrees. Plagioclase has good cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Continental crust, typically in stable cratonic areas, mountain belts, or areas of past magmatic activity. Dikes form in extensional or shear stress regimes where fractures allow magma ascent.

Key Facts

Hardness

Granite: 6-7 (Mohs, due to quartz and feldspar). Mafic Dike: 5-6 (Mohs, due to pyroxene, plagioclase).

Specific Gravity

Granite: 2.6-2.7 g/cm³. Mafic Dike: 2.8-3.1 g/cm³.

Crystal System

Granite: Monoclinic (feldspars), Hexagonal (quartz), Monoclinic (micas). Mafic Dike: Monoclinic (pyroxene, plagioclase), Orthorhombic (olivine).

Color

Granite: White, pink, gray, red. Mafic Dike: Dark gray to black.

Luster

Vitreous to dull.

Transparency

Opaque to translucent (individual crystals).

Fracture

Granite: Uneven to conchoidal (quartz). Mafic Dike: Uneven to sub-conchoidal.

Cleavage

Granite: Good in feldspars and micas. Mafic Dike: Good in pyroxenes and plagioclase.

Composition

Granite: Predominantly quartz (20-60%), alkali feldspar (35-90% of total feldspar), plagioclase, biotite, muscovite, hornblende. Mafic Dike: Predominantly plagioclase (calcic), pyroxene (augite), olivine, minor amphibole, biotite, magnetite.

Physical characteristics

• Crystal Habit: Granite: Granular, anhedral to subhedral. Mafic Dike: Fine-grained, sometimes porphyritic or ophitic.
• Cleavage Type: Granite: Two directions at ~90° (feldspar), perfect basal (mica). Mafic Dike: Two directions at ~90° (pyroxene), two directions (plagioclase).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to dull.

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