How to Analyze Kimberlite Samples in the Field

Kimberlite, a rare and fascinating igneous rock, is of significant interest to geologists and gemologists alike due to its association with diamonds. Found deep within the Earth’s mantle and transported to the surface through volcanic pipes, kimberlite provides critical clues about the geological processes occurring far beneath our feet. Analyzing kimberlite samples in the field is a vital first step towards understanding their composition, origin, and potential diamond content before conducting more detailed laboratory analyses.

This article will guide you through the methods and best practices for analyzing kimberlite samples in the field. Whether you are a geologist on a remote expedition or a student learning about igneous petrology, understanding how to effectively analyze kimberlite in situ will enhance your ability to interpret geological data and make informed decisions regarding sample collection.

Understanding Kimberlite: A Brief Overview

Before diving into field analysis techniques, it’s important to understand what makes kimberlite unique:

• Origin: Kimberlite is an ultramafic volcanic rock formed from magma originating deep within the Earth’s mantle, often at depths exceeding 150 kilometers.
• Composition: It typically contains olivine, phlogopite mica, pyroxenes, and various accessory minerals such as garnet and ilmenite. The presence of indicator minerals like chromian diopside and chromium spinel is also common.
• Texture: Kimberlites can exhibit a range of textures from coherent to fragmental (brecciated), often containing xenoliths or fragments of mantle rocks.
• Economic Importance: Kimberlites are the primary source rocks for diamonds, making their study crucial for diamond exploration.

Preparing for Field Analysis

Effective field analysis begins long before arriving at the sampling site. Preparation includes:

• Gathering Essential Tools: A hand lens (10x magnification), geologist’s hammer, GPS device, field notebook, sample bags, compass clinometer, portable XRF analyzer (if available), and safety gear.
• Familiarizing with Regional Geology: Understanding the local geological context helps in identifying kimberlite outcrops and potential sampling locations.
• Planning Logistics: Ensure accessibility to outcrop sites and arrange for proper sample transport methods.

Step 1: Locating Kimberlite Outcrops

Kimberlites are commonly exposed as pipes or dikes. When searching for these:

• Look for characteristic greenish-blue to dark gray rocks that often weather quickly.
• Identify volcanic neck structures or circular depressions indicative of kimberlite pipes.
• Use geological maps and remote sensing data to pinpoint likely kimberlite occurrences.

Step 2: Macroscopic Examination of Samples

Once you have located kimberlite outcrops or float boulders:

Observe Color and Weathering Patterns

Kimberlite often exhibits distinct coloration:

• Fresh surfaces may appear greenish due to abundant olivine.
• Weathered surfaces tend to be rusty orange-brown owing to iron oxidation.

Note the degree of weathering as it affects mineral visibility.

Examine Texture and Grain Size

Identify whether the rock is coherent or brecciated:

• Coherent kimberlite has an interlocking crystalline texture.
• Brecciated kimberlite consists of angular fragments cemented together.

Record grain size variations; finer-grained areas may suggest different cooling histories.

Identify Phenocrysts and Groundmass

Look for large crystals (phenocrysts) embedded within finer groundmass:

• Olivine phenocrysts are typically greenish and glassy.
• Phlogopite appears as shiny brown flakes.

Determining phenocryst abundance aids in classification of kimberlite types (e.g., hypabyssal vs. crater facies).

Step 3: Mineral Identification Using Hand Lens

A hand lens allows you to inspect individual mineral grains:

• Olivine: Glassy luster with green coloration; may show conchoidal fractures.
• Phlogopite: Brownish mica flakes that are flexible and shiny.
• Garnet: Usually red-brown with high relief under magnification.
• Ilmenite: Black opaque grains often magnetic.

Note the presence of mantle-derived xenoliths such as peridotite fragments – these may contain diamonds or diamondiferous inclusions.

Step 4: Field Tests for Mineral Properties

Conduct simple physical tests on accessible minerals:

• Hardness Test: Scratching minerals with a knife blade; olivine has hardness around 6.5–7 on Mohs scale.
• Magnetism Test: Use a small magnet to detect magnetic minerals like magnetite or ilmenite.
• Reaction to Acid: Apply dilute hydrochloric acid (if safe) to check for carbonate cement which can be present in altered kimberlites.

Record any fluid inclusions or vesicles indicating volatile content in the magma.

Step 5: Sample Collection Strategy

Proper sampling ensures representativeness and suitability for subsequent analysis:

• Collect fresh, unweathered rock specimens where possible.
• Take multiple samples from different parts of an outcrop to capture variability.
• Label samples clearly with location coordinates, orientation measurements, and notes on lithology.

Avoid contamination by using clean tools and bags for each specimen.

Step 6: Utilizing Portable Analytical Instruments

If available, portable instruments can greatly enhance field analysis accuracy:

Portable X-Ray Fluorescence (pXRF)

pXRF analyzers provide rapid elemental composition data on-site:

• Use pXRF to quantify concentrations of key elements such as magnesium, chromium, nickel, titanium, and calcium.
• Compare chemical signatures with known kimberlite compositions to confirm identification.

Portable Raman Spectrometer

Raman spectrometry can help identify mineral phases including carbon allotropes potentially related to diamonds.

Field Spectroscopy

Visible-Near Infrared (VNIR) spectroscopy may assist in identifying alteration minerals like serpentine or clay that form from kimberlite weathering.

Step 7: Documenting Geological Context

Accurate recording of field observations is essential:

• Sketch outcrop geometry and structural features such as joint patterns or flow banding.
• Note spatial relationships between kimberlite bodies and country rock formations.
• Record GPS coordinates precisely for georeferencing samples during later laboratory work.

Photograph samples with scale bars under good lighting conditions for reference.

Step 8: Interpreting Field Data

Based on your observations, classify the kimberlite according to established schemes:

• Distinguish between crater facies (fragmental material near surface) versus hypabyssal facies (coherent intrusive rocks).
• Assess whether mineralogy suggests primary kimberlitic magma or altered/weathered material.
• Identify potential indicator minerals that might hint at diamond presence.

Combine macroscopic observations with portable instrument data where available for robust interpretation.

Safety Considerations During Fieldwork

Field geology inherently carries risks; keep these points in mind when working with kimberlites:

• Wear appropriate personal protective equipment including helmets, goggles, gloves, and sturdy boots.
• Exercise caution when hammering rocks or climbing rugged terrain near pipes.
• Be aware of remote locations and communicate plans with your team regularly.

Conclusion

Analyzing kimberlite samples in the field is an indispensable skill for geologists involved in diamond exploration and research into mantle processes. By carefully observing physical properties such as color, texture, mineralogy, and chemical composition using both traditional tools and modern portable instruments, you gain valuable insights that guide further laboratory studies. Meticulous sample collection paired with detailed documentation enhances the quality of your geological investigations.

The complexity of kimberlite makes its study both challenging and rewarding. Mastery of field analysis techniques not only aids in identifying promising diamondiferous rocks but also contributes to broader understanding of Earth’s deep interior dynamics. With practice and patience, analyzing kimberlites can become a fascinating window into some of nature’s most extraordinary phenomena beneath our feet.