The Mineralogy of Kimberlite: What Makes It Unique?

Kimberlite is a fascinating and scientifically significant type of igneous rock, best known as the primary source of natural diamonds. Its unique mineralogy not only makes it an intriguing subject of study for geologists and mineralogists but also a key to understanding deep Earth processes. This article explores the mineralogy of kimberlite, detailing what makes it distinct from other igneous rocks and why it holds such importance in both geology and economic mineral exploration.

Introduction to Kimberlite

Kimberlite is a volatile-rich, ultramafic volcanic rock that originates from great depths within the Earth’s mantle—typically more than 150 kilometers below the surface. It is named after the town of Kimberley in South Africa, where the first significant diamond-bearing deposits were discovered in the late 19th century.

These rocks are often found as carrot-shaped pipes or dikes that have intruded through the Earth’s crust, bringing mantle material with them. Kimberlites are relatively rare but are critically important because they contain diamonds formed under high-pressure conditions deep within the mantle.

The Formation and Geological Context of Kimberlite

Kimberlites form from magma that is generated under extreme pressure and temperature conditions in the mantle. This magma is rich in volatiles such as carbon dioxide (CO2) and water (H2O), which lower its melting point and facilitate its rapid ascent through the lithosphere.

The ascent is typically rapid and explosive, producing brecciated rock structures and sometimes bringing fragments of mantle material—including peridotite xenoliths—toward the surface. This rapid transport allows diamonds, which crystallize at depths exceeding 140 kilometers under pressures over 45 kilobars, to survive and reach near-surface environments without converting into graphite or being destroyed.

Mineralogical Composition of Kimberlite

Primary Minerals

The mineralogical makeup of kimberlite is complex and variable due to differences in source composition, depth of melting, and alteration processes. However, several minerals are characteristic or diagnostic for kimberlite:

Olivine: The most abundant primary mineral in kimberlite is olivine [(Mg,Fe)2SiO4], a high-temperature silicate mineral common in mantle-derived rocks. In kimberlites, olivine often appears as large phenocrysts (large crystals set within a finer-grained matrix) that may be partially altered to serpentine or other secondary minerals.
Phlogopite: This dark mica [(KMg3(AlSi3O10)(OH)2)] is another abundant primary mineral. Phlogopite occurs as tabular crystals and contributes to the characteristic greenish color of some kimberlites.
Clinopyroxene: Specifically diopside or augite (CaMgSi2O6), clinopyroxenes are common constituents in kimberlite. They usually occur as euhedral to subhedral crystals within the fine-grained groundmass.
Carbonates: Kimberlites often contain carbonate minerals such as calcite (CaCO3) or dolomite (CaMg(CO3)2), reflecting their CO2-rich volatile content.
Ilmenite: This titanium-iron oxide (FeTiO3) is commonly found as small grains or inclusions within kimberlite, often associated with magnetite.
Perovskite: A rare titanium-calcium oxide mineral (CaTiO3) seen occasionally in kimberlites; it can be an indicator of specific mantle source characteristics.

Accessory and Indicator Minerals

Several accessory minerals serve as important indicators for identifying kimberlitic rocks:

Chromian Spinel: Spinels rich in chromium are often found in kimberlites and related mantle xenoliths. They provide clues about mantle oxidation state and composition.
Garnet: Pyrope garnet (Mg3Al2Si3O12), especially those rich in chromium, frequently occurs as xenocrysts or inclusions within kimberlites. These garnets are important diamond indicator minerals because their chemistry reflects deep mantle conditions conducive to diamond stability.
Magnetite: An iron oxide mineral that often coexists with ilmenite; it can form intergrowth textures with ilmenite known as “intergrowth exsolution lamellae.”
Diamond: While not a mineral exclusive to kimberlite, diamonds occur predominantly in these rocks due to their unique formation environment. Diamonds appear as rare but highly significant inclusions or xenocrysts within the kimberlite matrix.

Groundmass Minerals

The fine-grained matrix or groundmass of kimberlite typically consists of a mix of:

Serpentine group minerals derived from alteration of olivine.
Calcite or other carbonate minerals.
Clay minerals formed by weathering.
Other secondary silicates such as chlorite or amphibole developed during alteration stages.

What Makes Kimberlite Unique?

Depth of Origin

Kimberlites originate at greater depths than most other volcanic rocks. This deep mantle source—extending well into the lithospheric mantle beneath stable cratonic regions—distinguishes them from basaltic magmas derived from upper mantle sources at shallower depths.

Volatile Content

Kimberlites have an unusually high volatile content compared to typical mafic rocks. Their CO2 and H2O contents significantly influence their melting behavior, eruption style, and alteration patterns. The volatiles contribute to explosive volcanic activity and help preserve diamonds during transport.

Mineral Assemblage Linked to Mantle Conditions

The coexistence of olivine, phlogopite, chromian spinel, pyrope garnet, ilmenite, perovskite, and carbonates reflects a very specific set of pressure-temperature conditions within a chemically distinct mantle source domain. Notably:

The presence of phlogopite indicates potassium enrichment uncommon in many ultramafic rocks.
Carbonates suggest carbonate-rich melts or metasomatism.
Chromium-rich pyrope garnet points to deep lithospheric mantle environments conducive to diamond growth.

Indicator Minerals for Diamond Exploration

One reason kimberlites are extensively studied is their association with diamonds. The presence of certain indicator minerals—chromian pyrope garnet, chromian spinel, ilmenite with specific compositions—can indicate proximity to diamondiferous pipes before diamonds themselves are found. These minerals provide invaluable clues for prospecting geologists seeking new diamond deposits globally.

Textural Features

Kimberlites often exhibit distinctive textures including fragmented xenoliths embedded within a fine-grained matrix produced by explosive emplacement. Olivine phenocrysts show partial alteration textures due to interaction with volatile-rich fluids during emplacement. These textural characteristics help differentiate kimberlites from other ultramafic rocks such as lamproites or basalts.

Alteration and Weathering

Kimberlites undergo extensive post-emplacement alteration due to their volatile richness and exposure at Earth’s surface:

Primary olivine alters readily to serpentine group minerals under hydrothermal conditions.
Carbonate minerals may recrystallize or dissolve.
Clay minerals develop through weathering processes.

This alteration complicates both field identification and laboratory analysis but also results in distinctive geochemical signatures useful for exploration.

Economic Importance

Beyond their scientific interest, kimberlites hold major economic significance due primarily to their role as hosts for natural diamonds. Mining operations across South Africa, Russia, Canada, Botswana, Australia, and other countries target kimberlite pipes for their valuable gem-quality diamonds.

Understanding the mineralogy helps geologists identify potential new deposits by recognizing hallmark indicator minerals and assessing alterations that might affect diamond preservation.

Conclusion

Kimberlite’s unique mineralogy reflects its extraordinary origin from deep within Earth’s mantle—a domain inaccessible by any other means except through these volcanic rocks that bring fragments of ancient lithosphere to the surface. Its distinct assemblage rich in olivine, phlogopite, chromian spinel, garnet, carbonates, ilmenite, and rare phases such as perovskite sets it apart from all other igneous rocks.

This exceptional combination of mineralogical features not only makes kimberlite fascinating from a petrological perspective but also critically important for diamond exploration worldwide. As research continues on these enigmatic rocks, they will undoubtedly remain central to our understanding of Earth’s interior processes and resources for years to come.