How to Identify Kimberlite: A Comprehensive Guide for Rockhounds and Geologists
Kimberlite. The very name conjures images of sparkling diamonds and hidden treasures. This unique type of igneous rock is renowned as the primary host for diamonds, making its identification a prized skill for geologists, prospectors, and even avid rockhounds. However, identifying kimberlite isn’t always straightforward. Its appearance can vary significantly depending on weathering, alteration, and the presence of other minerals. This comprehensive guide will equip you with the knowledge and techniques needed to confidently identify kimberlite in the field and laboratory.
## What is Kimberlite?
Kimberlite is a rare, potassium-rich, ultramafic igneous rock that originates deep within the Earth’s mantle. It’s characterized by its volatile-rich nature, including carbon dioxide and water, which contribute to its explosive eruption style. These eruptions create characteristic volcanic pipes, also known as kimberlite pipes or diatremes. These pipes are cone-shaped structures that can extend hundreds of meters below the surface and are often filled with a mixture of kimberlite rock fragments, mantle-derived minerals, and crustal rocks.
Kimberlite is named after the town of Kimberley, South Africa, where the first diamond-bearing kimberlite pipe was discovered in the 1870s. Since then, kimberlite pipes have been found in various locations around the world, including Russia, Canada, Australia, and the United States.
## Why is Kimberlite Identification Important?
The primary reason for identifying kimberlite is its association with diamonds. While not all kimberlites contain diamonds (in fact, only a small percentage are economically viable), their presence strongly suggests the potential for diamond deposits. Accurate identification allows for targeted exploration efforts, saving time and resources. Furthermore, studying kimberlites provides valuable insights into the Earth’s mantle composition and processes.
Beyond diamond exploration, kimberlites are also studied for their unique mineral assemblages, which can provide clues about the conditions under which they formed. These rocks often contain rare and unusual minerals that are of interest to mineralogists and geochemists.
## Key Characteristics of Kimberlite
Before diving into the identification process, it’s crucial to understand the key characteristics that distinguish kimberlite from other rock types. These characteristics include:
* **Mineralogy:** Kimberlite is defined by a specific suite of minerals, often referred to as indicator minerals. These minerals are typically derived from the Earth’s mantle and include olivine, phlogopite mica, garnet (especially pyrope garnet), chromite, and ilmenite. Serpentine, calcite, and other secondary minerals are also common, especially in altered kimberlites.
* **Texture:** Kimberlite typically has a porphyritic texture, meaning it contains large crystals (phenocrysts) embedded in a fine-grained groundmass. The phenocrysts are often olivine and phlogopite, while the groundmass consists of serpentine, calcite, and other fine-grained minerals. Macrocrysts (large crystals, often greater than 1cm) of garnet, ilmenite and other mantle minerals are also common.
* **Color:** The color of kimberlite can vary significantly depending on its composition and alteration. Fresh kimberlite is often bluish-green or dark gray, but it can weather to brown, yellow, or reddish-brown. Highly altered kimberlite can even appear white or cream-colored.
* **Inclusions:** Kimberlite frequently contains inclusions of other rocks and minerals, known as xenoliths and xenocrysts, respectively. These inclusions are picked up by the kimberlite magma as it ascends through the Earth’s crust and can provide valuable information about the rocks the magma passed through. Common xenoliths include crustal rocks such as granite, gneiss, and sandstone, as well as mantle-derived rocks like peridotite and eclogite.
* **Geological Setting:** Kimberlites are typically found in stable continental regions, often associated with ancient cratons. They occur as vertical pipes or dikes that cut through the surrounding country rock. Kimberlite pipes are often clustered together in kimberlite fields.
## Detailed Steps for Kimberlite Identification
Now, let’s move on to the practical steps involved in identifying kimberlite. This process involves a combination of field observations, hand specimen analysis, and potentially laboratory testing.
**Step 1: Field Reconnaissance and Location**
* **Research Existing Geological Maps and Data:** Before heading into the field, thoroughly research the geology of the area you plan to explore. Consult geological maps, reports, and academic publications to identify areas where kimberlites have been previously reported or where the geological setting is favorable for their occurrence. Online databases and geological surveys are valuable resources.
* **Identify Potential Kimberlite Indicators:** Look for features that may indicate the presence of kimberlite, such as:
* **Circular or Oval Depressions:** Kimberlite pipes often erode to form subtle depressions in the landscape. These depressions may be filled with soil or vegetation, making them difficult to spot, but careful observation can reveal their presence. Use aerial photography and satellite imagery to help identify these features.
* **Anomalous Vegetation Patterns:** The unique soil chemistry associated with kimberlite can sometimes support distinct vegetation patterns. For example, certain plant species may thrive in kimberlite soils due to the higher levels of magnesium and other nutrients.
* **Concentrations of Indicator Minerals:** Even if the kimberlite itself is not exposed, indicator minerals like garnet, ilmenite, and chromite may be found in stream sediments or soil samples downstream from a kimberlite pipe. Panning for these minerals can be a useful prospecting technique.
* **Changes in Topography:** Sometimes Kimberlite will form subtle rises or knobs due to differential weathering.
* **Obtain Permission:** Always obtain permission from landowners or relevant authorities before conducting any exploration activities on their property.
**Step 2: Hand Specimen Analysis**
Once you’ve located a potential kimberlite outcrop or sample, carefully examine it using the following techniques:
* **Visual Inspection:**
* **Color:** Note the color of the rock. Is it bluish-green, dark gray, brown, yellow, or reddish-brown? Keep in mind that color can be misleading due to weathering.
* **Texture:** Examine the texture of the rock. Is it porphyritic, with large crystals embedded in a finer-grained matrix? Look for the presence of phenocrysts of olivine, phlogopite, and other minerals.
* **Inclusions:** Check for the presence of xenoliths and xenocrysts. Can you identify any fragments of other rocks or minerals within the kimberlite?
* **Alteration:** Assess the degree of alteration. Is the rock heavily weathered or altered to secondary minerals like serpentine and calcite?
* **Mineral Identification:**
* **Olivine:** Look for glassy, yellowish-green or olive-green crystals. Olivine is a common phenocryst in kimberlite.
* **Phlogopite:** Identify bronze-colored or brownish mica flakes. Phlogopite is another characteristic mineral in kimberlite.
* **Garnet:** Search for small, rounded, red or purple crystals. Pyrope garnet is a key indicator mineral for kimberlite. Be aware that other types of garnet exist and may not be indicative of kimberlite.
* **Ilmenite:** Look for black, metallic crystals with a submetallic luster. Ilmenite is another common indicator mineral.
* **Chromite:** Identify small, black, octahedral crystals. Chromite is often associated with kimberlite.
* **Serpentine:** Serpentine is a common alteration product in kimberlite and can appear as greenish or yellowish, waxy-looking material.
* **Calcite:** Calcite can be present as veins, patches, or as a component of the groundmass. It’s often white or colorless.
Use a hand lens or magnifying glass to help identify the minerals. A streak plate can be used to determine the streak color of minerals like chromite and ilmenite.
* **Hardness Test:** Perform a simple hardness test to help identify the minerals. Olivine and garnet are relatively hard minerals (6.5-7.5 on the Mohs hardness scale), while serpentine is much softer (2.5-4). Use a scratch test kit or common objects like a steel nail (hardness 5.5) to estimate the hardness of the minerals.
* **Acid Test:** If you suspect the presence of calcite, perform an acid test. Place a drop of dilute hydrochloric acid (HCl) on the sample. If calcite is present, it will fizz or effervesce.
**Step 3: Detailed Examination of Indicator Minerals**
Kimberlite indicator minerals (KIMs) are a crucial tool in kimberlite exploration. Identifying these minerals, even in the absence of intact kimberlite rock, can point towards the proximity of a kimberlite source. Pay special attention to the following:
* **Garnet (Pyrope):** Pyrope garnet is a magnesium-rich garnet variety that is strongly associated with kimberlite. It typically has a deep red to purple color and a characteristic chemical composition. Look for garnets with a high magnesium content and low calcium content. Certain surface textures on garnets, like ‘kelyphite rims’ (alteration rims), are also indicative of mantle origin.
* **Ilmenite:** Kimberlitic ilmenite is often magnesium-rich and can be distinguished from other types of ilmenite by its chemical composition and trace element content. Look for ilmenite grains that are rounded and have a metallic luster.
* **Chromite:** Kimberlitic chromite is typically chromium-rich and magnesium-rich. It often occurs as small, black, octahedral crystals. Look for chromite grains with a high chromium to iron ratio.
* **Olivine:** Though common in many rock types, olivine in kimberlites is typically forsteritic (magnesium-rich). Its presence in conjunction with other KIMs supports kimberlite identification.
* **Diopside:** Chrome diopside, a bright green variety of diopside, is also sometimes found in kimberlites.
**Step 4: Microscopic Analysis (if available)**
If you have access to a petrographic microscope, thin sections of the kimberlite sample can be examined to confirm the mineral identification and textural characteristics. Microscopic analysis can reveal features that are not visible to the naked eye, such as the fine-grained groundmass texture, the presence of alteration minerals, and the composition of the xenoliths.
* **Identify Groundmass Minerals:** Use the microscope to identify the minerals that make up the groundmass of the kimberlite. Common groundmass minerals include serpentine, calcite, phlogopite, and perovskite.
* **Examine Phenocrysts and Xenocrysts:** Analyze the composition and texture of the phenocrysts and xenocrysts. Look for signs of reaction rims or alteration around the crystals.
* **Determine Textural Relationships:** Observe the textural relationships between the different minerals. For example, look for evidence of olivine crystals being replaced by serpentine or calcite.
**Step 5: Geochemical Analysis (Laboratory Testing)**
For a definitive identification of kimberlite, geochemical analysis is often necessary. This involves sending a sample to a laboratory for chemical analysis to determine its elemental composition. Key elements to analyze include:
* **Major Elements:** Analyze the major element composition of the rock, including SiO2, TiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and P2O5. Kimberlites typically have low SiO2 and high MgO contents.
* **Trace Elements:** Analyze the trace element composition of the rock, including elements like Cr, Ni, Nb, Zr, and REE (Rare Earth Elements). Kimberlites typically have high Cr and Ni contents and a characteristic REE pattern.
* **Indicator Mineral Geochemistry:** If indicator minerals are present, analyze their chemical composition using techniques like electron microprobe analysis (EMPA) or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This can help confirm their mantle origin and distinguish them from similar minerals from other rock types.
The geochemical data can be compared to published data on kimberlites from other locations to confirm the identification. Specialized geochemical plots, such as those using ratios of trace elements, can be used to distinguish kimberlites from other ultramafic rocks.
**Step 6: Isotopic Analysis (Advanced)**
In some cases, isotopic analysis may be necessary to further confirm the identification of kimberlite and to study its origin. Isotopic analysis involves measuring the ratios of different isotopes of elements like strontium (Sr), neodymium (Nd), and lead (Pb). These ratios can provide information about the age of the kimberlite and the source of the magma.
**Step 7: Contextual Analysis and Expert Consultation**
Even with geochemical and isotopic data, contextual analysis remains important. Consider the regional geology, the proximity to known kimberlite fields, and the overall geological setting. Consult with experienced geologists or mineralogists who specialize in kimberlite exploration for their expert opinion. Their experience can be invaluable in interpreting the data and making a final determination.
## Common Pitfalls and Challenges
Identifying kimberlite can be challenging due to several factors:
* **Weathering and Alteration:** Weathering and alteration can significantly alter the appearance of kimberlite, making it difficult to recognize. Alteration products like serpentine and calcite can obscure the original mineralogy and texture of the rock.
* **Lack of Exposure:** Kimberlite pipes are often covered by soil or vegetation, making it difficult to find fresh rock samples. Prospecting techniques like soil sampling and stream sediment analysis may be necessary to locate kimberlite indicators.
* **Similarity to Other Rocks:** Kimberlite can sometimes resemble other ultramafic rocks, such as lamproite and peridotite. Careful examination of the mineralogy, texture, and geochemistry is necessary to distinguish kimberlite from these other rock types.
* **Presence of Multiple Kimberlite Phases:** Some kimberlite pipes contain multiple phases of kimberlite, each with slightly different compositions and textures. This can make identification more complex.
## Useful Tools and Equipment
To effectively identify kimberlite, the following tools and equipment are recommended:
* **Geological Hammer:** For collecting rock samples.
* **Hand Lens or Magnifying Glass:** For examining minerals and textures.
* **Streak Plate:** For determining the streak color of minerals.
* **Acid Bottle with Dilute Hydrochloric Acid (HCl):** For testing for the presence of calcite.
* **Sample Bags and Notebook:** For collecting and labeling samples.
* **Geological Compass:** For determining the orientation of rock structures.
* **GPS Device:** For recording the location of samples.
* **Camera:** For documenting the geology of the area.
* **Safety Glasses:** For protecting your eyes from flying rock fragments.
* **Petrographic Microscope:** For detailed mineral identification (if available).
* **Sample Preparation Equipment:** For preparing thin sections for microscopic analysis (if available).
## Conclusion
Identifying kimberlite requires a combination of knowledge, skill, and patience. By understanding the key characteristics of kimberlite, following the detailed steps outlined in this guide, and utilizing the appropriate tools and equipment, you can significantly increase your chances of successfully identifying this valuable rock type. Remember to always practice responsible and ethical exploration techniques, and to consult with experts when needed. Happy rockhounding!