Kimberlite rock identification is key for both geologists and those interested in unique landscaping materials, and at rockscapes.net, we help you understand this rare, often diamond-bearing rock, distinguishing it from other igneous rocks and finding the perfect stones for your projects. With our guidance, you’ll confidently identify kimberlite by examining its color, texture, and composition, plus learn about its significance and where to find it. Discover how to spot kimberlite and explore the beauty it can bring to your landscape with valuable information about rock identification, landscape design, and natural stone supplies.
1. What is Kimberlite Rock?
Kimberlite, a rare type of potassic to ultrapotassic ultramafic rock, is characterized by its inequigranular texture and frequent presence of xenoliths and xenocrysts. It is named after Kimberly, South Africa, where it was first described, it holds significant geological interest and economic value due to its association with diamonds. According to research from Arizona State University’s School of Earth and Space Exploration, in July 2025, kimberlite formations provide valuable insights into the Earth’s mantle composition.
- Ultramafic Rock: Kimberlite is composed of very low silica content and high levels of magnesium and iron.
- Inequigranular Texture: The texture is marked by a mix of large and small crystals, known as macrocrysts and megacrysts, embedded in a fine-grained groundmass.
- Xenoliths and Xenocrysts: These are foreign rock fragments and crystals incorporated into the kimberlite during its ascent, often originating from the Earth’s mantle.
- Diamond Association: Kimberlite is one of the primary host rocks for diamonds, making its identification economically important.
2. What are the Key Properties and Quick Facts About Kimberlite Rocks?
Understanding the basic facts and properties of kimberlite rock can greatly assist in its identification. These include its origin, texture, color, chemical composition, hardness, and other physical characteristics.
| Property | Description |
|---|---|
| Rock Type | Igneous |
| Origin | Plutonic or intrusive |
| Subcategory | Peridotite |
| Associated Rocks | Eclogites and carbonatites |
| Texture | Inequigranular or porphyritic, rarely poikilitic |
| Color | Slate blue, blue-green, dark-gray, or black when unaltered; yellowish when weathered |
| Chemical Composition | Ultramafic or ultrabasic; rich in magnesium oxide and poor in silica |
| Mohs Hardness | 6-7 |
| Density | 2700-2900 kg/m3 |
| Magnetism | Weakly magnetic |
| Tectonic Environment | Continental intraplate |
| Age | Precambrian to Cretaceous (2.5Ga to 60-100 Ma) |
3. How Do You Identify Kimberlite in the Field?
Identifying kimberlite in the field involves observing its color, texture, and the presence of specific minerals. While these visual cues are helpful, confirming its identity and diamond-bearing potential often requires laboratory analysis.
- Color: Fresh kimberlite typically has a bluish or greenish tint. Miners often refer to unaltered kimberlite as “blue ground”. Weathered kimberlite tends to be yellowish and is known as “yellow ground”.
- Texture: Kimberlite features an inequigranular texture, meaning it contains a mix of large and small crystals. Look for large, angular to rounded macrocrysts (0.5-10mm) and megacrysts (1-20cm) set in a fine-grained groundmass.
- Brecciation: Kimberlite is often brecciated, meaning it consists of angular fragments of rock cemented together. This is especially common in kimberlite pipes.
- Magnetism: Kimberlite is weakly magnetic due to the presence of magnetite. You can test its magnetism using a magnet. Heating altered kimberlite may make it more magnetic due to the oxidation of limonite.
- Mineral Composition: Look for minerals such as olivine, phlogopite, diopside, garnet, magnesium-ilmenite, and magnesium chromite.
- Send for Analysis: If you suspect a rock is kimberlite, send a sample to a lab for thin section and geochemical analysis. This will provide definitive identification and determine if it is diamond-bearing.
4. What are Kimberlite’s Chemical and Mineral Composition?
Understanding the chemical and mineral composition of kimberlite helps in its accurate identification and classification. Kimberlite is characterized by high magnesium content, low silica, and the presence of specific minerals.
4.1 What is the Chemical Composition of Kimberlite?
Kimberlite is a potassic to ultrapotassic, magnesium-rich ultramafic rock with a unique chemical signature. Its composition includes:
- High MgO: Kimberlite contains 15-27 wt. % MgO, making it magnesium-rich.
- Low SiO2: It has a low silica content, ranging from 20-36 wt.% SiO2.
- Rich in Volatiles: Kimberlite is rich in volatiles, incompatible elements, and rare Earth elements (REE).
- Iron and Calcium Oxides: It contains considerable amounts of iron and calcium oxides.
- Low Al2O3: Kimberlite is low in aluminum oxide relative to alkali oxides.
- Potassic: It is potassic, meaning K2O > Na2O, and ultrapotassic, with a K2O to Na2O ratio of more than 3.
4.2 What is the Mineral Composition of Kimberlite?
The mineral composition of kimberlite is complex, with variations depending on whether it is Group I or Group II kimberlite. Key minerals include:
- Olivine: Typically anhedral to rounded macrocrysts.
- Phlogopite: Common macrocrysts, especially in Group II kimberlites.
- Diopside: Present as megacrysts or macrocrysts.
- Garnet: Commonly found as macrocrysts.
- Magnesium-Ilmenite and Magnesium Chromite: Also present as macrocrysts.
- Groundmass Minerals: Finer-grained minerals such as olivine, phlogopite, spinel, apatite, rutile, perovskite, and primary carbonates.
5. What are the Different Groups and Types of Kimberlite Rock?
Kimberlites are divided into two main groups: Group I and Group II. These groups differ in isotopic composition, mineralogy, and volatile content, reflecting distinct origins and formation processes.
5.1 What are Group I Kimberlites?
Group I kimberlites, also known as archetypal kimberlites, are characterized by:
- Potassic and Ultramafic: These rocks are potassic and ultramafic with CO2 as the predominant volatile.
- Macrocrysts and Megacrysts: They contain macrocrysts and megacrysts, including xenocrysts.
- Dominant Mineral: Anhedral olivine is the dominant mineral (except in fractionated types).
- Other Minerals: They may contain pyrope, magnesium-ilmenite, phlogopite, enstatite, titanium-poor chromite, and diopside.
- Groundmass: The groundmass consists of euhedral-to-subhedral olivine, monticellite, perovskite, spinel, apatite, or serpentine.
- Accessory Minerals: Accessory minerals include nickeliferous sulfides and rutile.
- Secondary Minerals: Serpentine and calcite are common secondary minerals, replacing monticellite, apatite, olivine, and phlogopite.
- Xenoliths: These rocks contain xenolithic macrocrysts such as forsterite, almandine-pyrope, chromium-diopside, chromium-pyrope, phlogopite, and magnesium-diopside.
5.2 What are Group II Kimberlites (Orangeites)?
Group II kimberlites, also known as micaceous or lamprophyritic kimberlites, are distinguished by:
- Ultrapotassic and Peralkaline: These rocks are ultrapotassic (K/Na > 3) and peralkaline, with water as the dominant volatile.
- Age: Their age is Cretaceous to early Jurassic.
- Dominant Mineral: Phlogopite macrocrysts and microphenocrysts are the dominant minerals, with lesser quantities of rounded olivine macrocrysts.
- Groundmass: The groundmass consists of tetraferriphlogopite to phlogopite.
- Primary Minerals: Primary minerals in the groundmass include diopside, spinel, REE-rich and strontium-rich perovskite, and strontium-rich apatite.
- Other Minerals: Other minerals include REE-rich phosphate, potassium-barium titanate, niobium-bearing rutile, and manganese-bearing ilmenite.
- Late-Stage Minerals: Late-stage groundmass contains zirconium silicate (zircon, wadeite, calcium-zirconium silicate, and kimzeyitic garnet).
- Evolved Members: Evolved members contain potassium richterite and sanidine.
- Secondary Minerals: Barite is the diuretic secondary mineral.
6. How are Kimberlite and Diamonds Related?
Kimberlite is the primary host rock for diamonds. These precious gemstones are found both in weathered sediments and unaltered rock. Diamonds are macro to megacrysts xenoliths in kimberlites, meaning they are inclusions that are older than the kimberlite itself. Kimberlite melts pick up diamonds as they rise to the surface.
Diamond in kimberlite matrix
- Origin: Diamonds originate from depths of at least 150 km, where pressure exceeds 4.0 GPa.
- Inclusions: Inclusions in diamonds, such as peridotites, eclogitic materials, magnesium-perovskite, and ferropericlase, indicate formation at great depths, sometimes exceeding 670 km.
- Occurrence: Diamond-bearing kimberlites are mainly found on Archean cratons and Proterozoic mobile belts on the craton’s margins.
- Locations: Diamond-bearing kimberlites occur in various countries, including the US (Arkansas, Wyoming, and Colorado), South Africa, Russia, DRC Congo, Brazil, China, Botswana, Canada, Zimbabwe, Angola, and Australia.
- Concentration: Diamonds are present in kimberlite in meager amounts, typically 0.25-5.7 carats per ton (0.05-1.14 g/ton) of rock. Only a small percentage of kimberlites (less than 1%) contain diamonds.
- Mining: Diamond mining is primarily conducted via opencast methods, with deep mining as an alternative when conditions prevent opencast mining.
7. Where and How Does Kimberlite Form?
Kimberlite formation is a complex process that occurs deep within the Earth’s mantle. These rocks are primarily found on thick, ancient continental cratons that have remained stable since the Precambrian era.
7.1 What are the Facies of Kimberlite?
Kimberlite rocks exhibit three main facies, each representing a different stage in the formation and eruption process:
- Hypabyssal Facies: These kimberlites are massive, unaltered blocks with a macrocryst texture. Their intrusion is non-explosive and may be associated with domes or swell structures.
- Diatreme Facies: These contain angular to rounded clasts, lapilli, and fragments of altered kimberlites (olivine and pyroxene have undergone partial to full serpentinization) and xenoliths. This facies represents material that fell back into the pipe during the eruption.
- Crater Facies: These facies contain brecciated kimberlite, including ash and xenoliths from country rock formed during the eruption, as well as lake sediments. The sediments are often layered and originate from reworked deposited kimberlite tuff and wall rock.
7.2 What are Kimberlite Pipes or Diatremes?
Kimberlite pipes are subterranean funnel or carrot-shaped geological structures filled with breccia. Most are overlain by maars (shallow crater lakes), taper downward at 80-85°C, and extend as deep as 300-400m below the surface. They terminate at root zones with irregularly shaped intrusions that transition to hypabyssal facies.
- Formation: These pipes form from explosive, near-surface eruptions. Rapidly rising and expanding gas-charged magma creates an upward flaring or conical explosion crater by tearing the surrounding country rock. The resulting fragmental materials fall back into the crater, and others form a tuff ring from pyroclasts.
- Lava Flows: Kimberlitic pipes rarely have lava flows, except in locations like Igwisi Hills, Tanzania.
- Size: Kimberlite pipes typically range from 50-500 m on the surface and narrow downwards. Their size depends on depth and magma quantities.
- Arrangement: These pipes may occur alone or clustered along elongated areas. Some coalesce at depths, while others connect to one or more dikes or have dikes on their roots.
- Erosion: Over millions of years, kimberlite eruptions may erode hills or mountains due to near-surface processes. The rocks are relatively soft and susceptible to erosion.
7.3 What are Kimberlite Dikes?
Dikes are non-fragmental kimberlites with a lenticular section and plan, including ring dykes. They can be single or a swarm, some parallel, and form from a single or many melt injection phases. These dikes are usually 1-3 meters thick but can occur over extensive lengths. Most dikes pinch towards the surface and thicken with depth. Some form pods or blows, which are lenticular enlargements near their top that can be 10-20 times thicker and up to 100 meters long. Some dikes may be root zones for diatremes.
7.4 What are Kimberlite Sills?
Sills, like dikes, consist of massive (non-fragmental) kimberlites and can be several hundred meters thick. They are less common, possibly because they are only visible if erosion coincides with the injection level.
7.5 How is Kimberlite Magma Formed?
Kimberlite magma forms from the partial melting of mantle peridotites, especially carbon-bearing, hydrous lherzolite or other peridotites below cratons. This partial melting occurs when volatiles (CO2 and H2O) reduce the mantle solidus temperature, generating kimberlitic melt. Prevailing oxygen fugacity favors the presence of stable magnesite (MgCO3) and dolomite (CaMg(CO3)2) minerals, which rapidly decompose as the magma rises.
- Magma Origin: Kimberlite magma originates in the mantle at depths of at least 150 km.
- Xenoliths and Minerals: The presence of xenoliths like diamonds and mantle peridotites supports the theory of deep origin, as these materials are not typical in the Earth’s crust.
- Ascent Speed: Kimberlite melts ascend quickly, preventing diamonds from reverting to graphite.
- Characteristics: These melts are volatile-rich (CO2 and H2O) and lack a glass matrix, which is common in rapidly quenched volcanic glasses.
7.6 How is Kimberlite Emplaced?
Emplacement of kimberlite magma involves several theories, including explosive volcanism, fluidization, hydrovolcanic activity, and the embryonic pipe theory. Generally, gas-charged magma rapidly ascends through weaknesses or fissures in the continental crust, milling xenoliths and megacrysts from the surrounding rock. Carbon dioxide is exsolved from the melt during ascent due to pressure drops.
- Alternative Theory: Russell et al. (2012) proposed that kimberlite forms from carbonatitic melts with nearly isometric carbon content. These carbonatite magmas have high CO2 and H2O solubility, giving the melt buoyancy for rapid ascent.
8. Where Can Kimberlite Be Found?
Kimberlite occurrences are known worldwide, but only a small fraction are diamondiferous, and even fewer have commercially viable diamond deposits.
8.1 Where is Kimberlite Found in the United States?
In the United States, kimberlite is found in several states:
- Arizona
- Arkansas
- Colorado
- Kansas
- Michigan
- Montana
- New York
- Pennsylvania
- Tennessee
- Utah
- Virginia
- Wisconsin
- Wyoming
8.2 Where is Kimberlite Found Around the World?
Globally, kimberlite is found in numerous countries, including:
- Angola
- Australia
- Bolivia
- South Africa
- Botswana
- Brazil
- Canada
- China
- DR Congo
- Eswatini
- Finland
- Greenland
- India
- Ireland
- Israel
- Ivory Coast
- Lesotho
- Namibia
- Pakistan
- Russia
- Solomon Islands
- Sweden
- Tanzania
- Ukraine
- Venezuela
- Zimbabwe
9. Why is Kimberlite Significant?
Kimberlites are significant for several reasons:
- Diamond Hosting: They are the main diamond-hosting rocks, providing a source of highly valued gemstones.
- Mantle Insights: They provide geologists and scientists with valuable information about the subcontinental mantle.
- Xenolith Studies: Studying xenoliths like diamonds, lherzolite, and harzburgite can give insights into the mantle composition.
- Geological Processes: Kimberlites offer information about processes and conditions (temperature and pressure) deep within the Earth without the need for drilling to such depths.
10. How Can You Identify Kimberlite Locations?
Identifying kimberlite locations involves looking for indicator minerals and specific plant species.
10.1 What are Kimberlite Indicator Minerals?
The primary method for identifying kimberlite pipe locations is by looking for kimberlite indicator minerals. These dense minerals are resistant to weathering and include:
- Chrome diopside
- Pyrope garnet
- Magnesium olivine ilmenite
These minerals can be found in heavy mineral concentrates in streams, rivers, or glacial tills. Colorful minerals like purple-red pyrope (Mg-rich garnet) and green chromium-rich clinopyroxene are common in type I kimberlites and can serve as indicator minerals.
10.2 Are There Any Botanical Indicators for Kimberlite?
A study by Haggerty (2014) suggests that Pandanus candelabrum, a palm-like plant, may serve as a botanical indicator for kimberlite pipes. This plant is known to grow only in these locations, possibly due to the unique mineralogy of the soils on these pipes.
11. How Do Kimberlites and Lamproites Differ?
Kimberlites and lamproites share some similarities as diamond-hosting rocks with depleted dunites and harzburgites, but they also have several key differences:
| Feature | Kimberlite | Lamproite |
|---|---|---|
| Dominant Volatile | CO2 | H2O |
| Silica Content | Silica-undersaturated minerals | Silica-saturated minerals |
| Location | Continental cratons | Adjacent Proterozoic belts |
| Morphology | Pipes that go deep with no lava flow | Subvolcanic bodies with lava flow |
| Accessory Minerals | Does not contain priderite and wadeite | Contains priderite and wadeite accessory minerals |
| Composition | Less titanium, rubidium, zirconium, barium | More titanium, rubidium, zirconium, barium |
FAQ: Identifying Kimberlite Rock
What is the primary way to identify kimberlite in the field?
The primary way to identify kimberlite in the field is by observing its color (bluish or greenish when fresh, yellowish when weathered), texture (inequigranular with macrocrysts and megacrysts in a fine-grained groundmass), and brecciation (angular fragments cemented together).
Are all kimberlites diamond-bearing?
No, not all kimberlites are diamond-bearing. In fact, only a small percentage (less than 1%) of known kimberlite occurrences contain commercially viable diamond deposits.
What are the two main groups of kimberlite, and how do they differ?
The two main groups of kimberlite are Group I and Group II. Group I kimberlites are potassic and ultramafic, with CO2 as the predominant volatile, while Group II kimberlites (orangeites) are ultrapotassic and peralkaline, with water as the dominant volatile. They also differ in mineral composition and isotopic ratios.
Where do diamonds in kimberlite come from?
Diamonds in kimberlite are xenoliths, meaning they originated deep within the Earth’s mantle (at least 150 km) and were transported to the surface by kimberlite magma. The diamonds are older than the kimberlite itself.
What is the significance of kimberlite indicator minerals?
Kimberlite indicator minerals, such as chrome diopside, pyrope garnet, and magnesium olivine ilmenite, are dense and resistant to weathering. They help in locating kimberlite pipes by tracing their presence in streams, rivers, and glacial tills.
Can plants indicate the presence of kimberlite?
Yes, a recent study suggests that Pandanus candelabrum, a palm-like plant, may serve as a botanical indicator for kimberlite pipes due to its preference for the unique mineralogy of soils found in these locations.
How deep do diamonds form in the Earth’s mantle?
Diamonds form at depths of at least 150 kilometers (93 miles) in the Earth’s mantle, where the pressure is high enough to stabilize the carbon atoms in a diamond structure. Some diamonds may even form at depths exceeding 670 kilometers (416 miles).
Why is kimberlite called a “complex hybrid” rock?
Kimberlite is often described as a “complex hybrid” rock because it is impossible to determine the exact amount of cumulates and xenoliths incorporated into the original melt. This makes it difficult to ascertain the original composition of the magma.
What are the key differences between kimberlites and lamproites?
Kimberlites and lamproites differ in their dominant volatiles (CO2 vs. H2O), silica content (undersaturated vs. saturated), location (continental cratons vs. adjacent Proterozoic belts), and morphology (pipes vs. subvolcanic bodies with lava flows).
How can I use kimberlite in landscape design?
Kimberlite can add a unique geological element to landscaping due to its distinct colors and textures. Depending on its composition, it can provide an ultramafic touch to gardens, rock gardens, and decorative stone arrangements.
Ready to explore the beauty and potential of kimberlite in your landscape? Visit rockscapes.net for a wide selection of natural stones, expert advice, and innovative design ideas. Contact us at 1151 S Forest Ave, Tempe, AZ 85281, United States or call +1 (480) 965-9011. Let us help you bring your vision to life!
