Diamond in Kimberlite

Description

Diamond in Kimberlite refers to the occurrence of natural diamonds (crystalline carbon) embedded within a kimberlite matrix. Kimberlite is a dark, often greenish-blue to black, porphyritic igneous rock characterized by a distinctive texture with phenocrysts (larger crystals) of olivine, phlogopite, pyroxene, and garnet set in a fine-grained groundmass. The diamonds themselves are typically small, often anhedral to euhedral crystals, ranging from microscopic to several carats in size, and can be colorless, yellow, brown, or rarely other colors. The kimberlite matrix often shows signs of alteration due to its volatile-rich nature and interaction with groundwater during emplacement.

How to Identify

Color

Kimberlite matrix is typically dark green, blue-green, or black, often weathering to brown or yellowish-brown. Diamonds within can be colorless, yellow, brown, or rarely other hues.

Luster

Kimberlite matrix is dull to earthy, sometimes greasy. Diamonds exhibit an adamantine (diamond-like) luster.

Texture

Kimberlite is porphyritic, with larger crystals (phenocrysts) of olivine, phlogopite, and garnet embedded in a fine-grained groundmass. Diamonds are typically discrete crystals within this matrix.

Crystal Form

Diamonds often occur as euhedral to subhedral octahedra, dodecahedra, or combinations thereof, sometimes with rounded or etched surfaces. Kimberlite minerals show various crystal forms depending on the mineral species.

Cleavage

Diamonds possess perfect octahedral cleavage ({111}). Kimberlite minerals have variable cleavage (e.g., olivine has poor cleavage, phlogopite has perfect basal cleavage).

Geological Environment

Kimberlite occurs in diatremes (pipes), dikes, and sills, typically found in stable cratonic regions of ancient continental crust. These structures represent volcanic conduits that brought mantle material to the surface.

Key Facts

• Hardness: Diamond: 10 (Mohs scale); Kimberlite minerals vary (e.g., olivine 6.5-7, phlogopite 2.5-3).
• Specific Gravity: Diamond: 3.50-3.53 g/cm³; Kimberlite: 2.5-2.9 g/cm³ (variable depending on alteration and mineralogy).
• Crystal System: Diamond: Isometric (cubic); Kimberlite: Polymineralic, individual minerals have their own crystal systems.
• Color: Diamond: Colorless, yellow, brown, rarely pink, blue, green; Kimberlite: Dark green, blue-green, black, weathering to brown/yellow.
• Luster: Diamond: Adamantine; Kimberlite: Dull, earthy, sometimes greasy.
• Transparency: Diamond: Transparent to opaque; Kimberlite: Opaque.
• Fracture: Diamond: Conchoidal to uneven; Kimberlite: Uneven to hackly.
• Cleavage: Diamond: Perfect octahedral ({111}); Kimberlite: Variable, depending on constituent minerals.
• Composition: Diamond: Pure carbon (C); Kimberlite: Ultramafic, potassic igneous rock composed primarily of olivine, phlogopite, serpentine, carbonates, and various accessory minerals (e.g., pyrope, chromite, ilmenite).

Quick Check

• Color: Kimberlite: Dark green to black; Diamond: Colorless, yellow, brown.
• Luster: Kimberlite: Dull to earthy; Diamond: Adamantine.
• Streak: Kimberlite: White to pale green/brown (due to alteration); Diamond: Colorless (no streak).

Physical Characteristics

• Crystal Habit: Diamond: Octahedral, dodecahedral, cubic, or combinations, often rounded or etched. Kimberlite: Porphyritic, with phenocrysts of various minerals.
• Cleavage Type: Diamond: Perfect octahedral ({111}); Kimberlite: Variable, depending on constituent minerals.
• Fracture Type: Diamond: Conchoidal to uneven; Kimberlite: Uneven to hackly.
• Tenacity: Diamond: Brittle; Kimberlite: Brittle.
• Luster Type: Diamond: Adamantine; Kimberlite: Dull, earthy, greasy.

Formation

Diamonds form under extreme pressure and high temperature conditions (typically 4.5-6 GPa and 900-1300 °C) at depths of 150-200 km within the Earth's mantle. They are subsequently brought to the surface rapidly (over hours to days) by explosive volcanic eruptions of kimberlite magma. Kimberlite is an ultramafic, potassic igneous rock that originates from deep within the mantle, ascending through conduits known as kimberlite pipes or diatremes. The rapid ascent prevents the diamonds from graphitizing.

Usage

Diamonds are primarily used as gemstones in jewelry due to their exceptional hardness, brilliance, and rarity. Industrial applications include cutting, grinding, drilling, and polishing tools, as well as in high-tech applications such as optics and electronics. Kimberlite itself is sometimes quarried for aggregate, but its primary economic significance lies in its potential to host diamonds.

Age Distribution

Kimberlites range in age from Proterozoic (approximately 1.2 billion years ago) to Cenozoic (approximately 50 million years ago), with most significant diamond-bearing kimberlites being Mesozoic (Jurassic to Cretaceous, 90-150 million years ago) or Proterozoic.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Diamond: Native Element; Kimberlite: Igneous Rock (Ultramafic, Potassic)

Group

Diamond: Carbon Group; Kimberlite: Kimberlite Group (a specific type of igneous rock)

Crystal System

Diamond: Isometric (Cubic); Kimberlite: Polymineralic, individual minerals have their own crystal systems.

Chemical Formula

Diamond: C; Kimberlite: Complex silicate, carbonate, and oxide composition (no single formula).

Composition

Diamond: Pure carbon; Kimberlite: Predominantly olivine, serpentine, phlogopite, calcite, dolomite, with accessory pyrope garnet, chromite, ilmenite, diopside, and perovskite.

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