Kimberlite is a rare igneous rock that originates deep within the Earth’s mantle. This rock is a variant of peridotite and is the primary host matrix for diamonds. Formed at depths between 150 and 450 kilometers, the magma is propelled to the surface through rapid, violent eruptions, preserving high-pressure minerals, including diamond. Identifying this unique rock requires systematic observation of its geological context, visual characteristics, and the presence of specific accompanying minerals.
Geological Setting and Occurrence
The presence of kimberlite is almost exclusively linked to cratons, the most ancient, stable parts of the continental crust. These vast, rigid blocks of lithosphere, often older than 2.5 billion years, provide the necessary stability for diamond formation and preservation. The kimberlite magma ascends from the mantle along deep fractures within these cratonic areas.
The most sought-after form of intrusion is the diatreme, a vertical, carrot-shaped structure known as a kimberlite pipe. These pipes widen near the surface and are the main target for diamond exploration, as they are conduits for explosively emplaced mantle material. Kimberlite can also occur as dikes and sills, which are sheet-like intrusions representing deeper root zones or lateral offshoots.
Visual Characteristics of the Kimberlite Matrix
The kimberlite matrix offers the first visual clues to its identity in the field. When fresh, the rock is typically dark-colored, appearing as a slate blue, greenish-gray, or black ultramafic rock. This color is due to the abundance of minerals like olivine, which is often altered by the magma’s volatile content.
Upon exposure to weathering and oxidation near the surface, the matrix changes significantly, becoming softer and altering to yellow or brown colors. This highly weathered and friable material is historically known as “yellow ground,” contrasting sharply with the less altered “blue ground” below. The characteristic texture is often porphyritic, meaning it contains larger, rounded crystals (macrocrysts) embedded within a fine-grained groundmass.
The matrix is also commonly brecciated, containing angular or rounded fragments of the surrounding country rock picked up during the explosive ascent. This fragmented appearance, combined with large crystals, gives the rock an inequigranular texture. Unlike many other hard igneous rocks, weathered kimberlite is relatively soft and crumbly, which aids in its identification.
Identifying Key Indicator Minerals
A definitive identification relies on finding specific heavy minerals, known as kimberlite indicator minerals, which were carried up from the mantle alongside diamonds. These minerals are denser and more resistant to weathering than the kimberlite matrix, causing them to concentrate in nearby soil or stream sediments. The presence of these distinctive minerals is a strong proxy for a kimberlite source.
Pyrope Garnet
Pyrope garnet is perhaps the most visually striking indicator, recognized by its rich, deep purple-red to violet color, which is distinct from the more common red garnets found in crustal rocks. These grains are often rounded and may exhibit an “orange-peel” surface texture due to chemical corrosion during transport. The color intensity is directly related to the high chromium content, a signature of the deep mantle environment.
Picroilmenite
Picroilmenite, a magnesium-rich form of ilmenite, appears as dense, black grains with a bright, metallic luster. These grains are typically rounded and highly reflective, often showing a glossy, mirror-like internal surface when fractured. The high iron content in picroilmenite gives it moderate to high magnetic susceptibility, allowing separation using a simple magnet.
Chrome Diopside
Chrome diopside presents as a brilliant emerald-green mineral, providing a sharp color contrast against the dark matrix or surrounding stream sediments. This mineral often has a glassy appearance and tends to break into blocky, rectangular grains because of its excellent cleavage. Since it is less resistant to weathering than garnet or ilmenite, finding it in a stream concentrate suggests a relatively close source.
Olivine Macrocrysts
The mineral olivine is the most abundant constituent of the kimberlite matrix, but its mantle-derived macrocrysts are also considered indicators. These crystals are typically yellow-green with a glassy luster, though they are often altered to soft, waxy serpentine minerals in the weathered rock. The presence of altered, serpentinized olivine macrocrysts is a consistent feature of kimberlite.
Next Steps for Sample Confirmation
Once a potential kimberlite sample has been visually identified in the field, a few basic tests can provide preliminary confirmation.
- Magnetism Test: The presence of picroilmenite means that a sample of the rock or its heavy mineral concentrate should exhibit a slight to moderate response to a strong magnet.
- Acid Test: Testing for the rock’s high carbonate content involves applying a drop of dilute hydrochloric acid. Mild effervescence, or bubbling, due to the reaction with secondary calcite supports the initial identification.
- Laboratory Analysis: For absolute certainty, definitive confirmation requires petrographic analysis of a thin section to study the mineralogy and texture. Indicator mineral grains are also subjected to microprobe analysis to confirm their specific mantle-derived chemical composition.