Black Granite

Description

Commercially, 'Black Granite' is a misnomer, as true granite is a felsic rock rich in quartz and feldspar, typically light-colored. The term 'Black Granite' in the stone industry refers to a variety of dark-colored intrusive igneous rocks, predominantly gabbro and diorite, but can also include anorthosite, diabase, and even fine-grained basalt. These rocks are characterized by their dark, often black or very dark gray, appearance due to a high content of mafic minerals like pyroxene, amphibole, and plagioclase feldspar (calcium-rich). They are coarse-grained to medium-grained, exhibiting a phaneritic texture.

How to Identify

Color

Typically very dark gray to black. Some varieties may show a greenish or bluish tint due to specific mineral compositions.

Luster

Vitreous to sub-vitreous, depending on the mineral constituents. Plagioclase feldspar often has a pearly luster on cleavage surfaces.

Texture

Phaneritic (coarse-grained) to medium-grained, with interlocking crystals visible to the naked eye. Some varieties (e.g., diabase) can be ophitic or subophitic.

Crystal Form

Anhedral to subhedral crystals, forming an interlocking mosaic. Individual mineral grains (e.g., pyroxene, plagioclase) may show characteristic crystal habits if well-formed.

Cleavage

Minerals within the rock (e.g., pyroxene, amphibole, plagioclase) exhibit distinct cleavage planes. Pyroxenes typically have two cleavages at nearly 90 degrees, amphiboles at approximately 56 and 124 degrees, and plagioclase has two cleavages at nearly 90 degrees.

Geological Environment

Forms in large intrusive bodies (plutons, batholiths, sills, dikes) within continental and oceanic crust, often associated with magmatic arcs, rift zones, and large igneous provinces.

Key Facts

• Hardness: 6-7 on the Mohs scale (due to constituent minerals like feldspar, pyroxene, amphibole).
• Specific Gravity: 2.8-3.3 g/cm³ (varies with mineral composition; gabbro is typically denser than diorite).
• Crystal System: Monoclinic (pyroxene, amphibole), Triclinic (plagioclase feldspar). The rock itself is an aggregate of crystals.
• Color: Dark gray to black.
• Luster: Vitreous to sub-vitreous.
• Transparency: Opaque (as a rock aggregate). Individual mineral grains can be translucent to opaque.
• Fracture: Irregular to conchoidal (individual mineral grains). The rock typically exhibits a blocky fracture.
• Cleavage: Present in constituent minerals (e.g., pyroxene, amphibole, plagioclase).
• Composition: Gabbro: Predominantly calcic plagioclase feldspar (>50%) and pyroxene (clinopyroxene, orthopyroxene), with minor olivine, amphibole, and iron-titanium oxides. Diorite: Predominantly plagioclase feldspar (andesine to oligoclase) and amphibole (hornblende), with lesser biotite, pyroxene, and quartz (up to 5%). Anorthosite: Composed almost entirely of plagioclase feldspar (>90%).

Quick Check

• Color: Very dark gray to black.
• Luster: Vitreous to sub-vitreous.
• Streak: White to very light gray (due to plagioclase and other lighter minerals, or the streak of individual mafic minerals is dark but diluted by other components).

Physical Characteristics

• Crystal Habit: Granular, interlocking anhedral to subhedral crystals.
• Cleavage Type: Good to perfect in constituent minerals (e.g., two directions in pyroxene and amphibole, two in plagioclase).
• Fracture Type: Irregular to blocky for the rock mass; conchoidal to uneven for individual mineral grains.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous.

Formation

Formed from the slow crystallization of magma deep beneath the Earth's surface (intrusive igneous rocks). Gabbro forms from mafic magma, rich in iron and magnesium, while diorite forms from intermediate magma. The slow cooling allows for the formation of large, interlocking crystals.

Usage

Primarily used as a dimension stone for countertops, flooring, wall cladding, monuments, and paving. Its durability, resistance to weathering, and aesthetic appeal make it highly desirable in construction and decorative applications.

Age Distribution

Ranges from Precambrian to Cenozoic, depending on the specific rock type (Gabbro, Diorite, etc.).

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous Rock (Plutonic)

Group

Mafic to Intermediate Intrusive Rocks

Crystal System

Polycrystalline aggregate; constituent minerals have their own crystal systems (e.g., monoclinic, triclinic).

Chemical Formula

No single chemical formula, as it's a rock composed of multiple minerals.

Composition

Gabbro: SiO₂ (45-52 wt%), Al₂O₃ (15-20 wt%), FeO+Fe₂O₃ (8-12 wt%), MgO (6-12 wt%), CaO (8-12 wt%), Na₂O (2-4 wt%), K₂O (<1 wt%). Diorite: SiO₂ (52-63 wt%), Al₂O₃ (15-18 wt%), FeO+Fe₂O₃ (3-7 wt%), MgO (2-5 wt%), CaO (4-8 wt%), Na₂O (3-5 wt%), K₂O (1-3 wt%).

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