KIM Report

Glossary of Geologic Terms

Posted by David

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1. aeromagnetic

Aeromagnetic surveys are a geophysical remote sensing technique that employ helicopter or airplane magnetometers to measure the variance in the Earth's magnetic field caused by near surface geology. Different rocks have different minerals that can have different magnetic properties. Kimberlitic rocks typically have a significantly different magnetic signature than common rock types found at the Earth's surface. These show up as magnetic anomalies on the survey map.

2. beryl

A cyclosilicate with a hexagonal structure containing beryllium. Gem-quality beryl is termed depending on its colour: green (emerald), pink (morganite), yellow (heliodor), blue-green (aquamarine), etc. The colour of the beryl depends on the nature of the impurities in the crystal structure (as with corundum): V, Cr, Fe, etc.

3. c/t

Carats per metric ton. One of the two main ways that diamond grade is expressed. The other is cpht: carats per one hundred tons.

4. carat

The unit of weight by which diamonds are classified. One carat equals 0.2 grams. This terminology is derived from the carob seed. This was a seed of fairly constant mass and they were used as a counterweight when weighing gemstones in pre-modern times.

5. carbonatite

This is an igneous rock characterized by an abundance of carbonate (CO3) minerals (e.g. calcite, dolomite, siderite, etc.). This is in contrast to the vast majority of igneous rocks that are dominated by silicate (SiO4) minerals (quartz, feldspar, olivine, pyroxene, mica, etc.). Carbonatites are relatively rare, but can contain minerals that are important sources of rare earth elements (REEs), niobium, tantalum, gold, and uranium. These rocks can also share some petrologic similarities with kimberlites and pegmatites.

6. corundum

Naturally occurring Al2O3 in a hexagonal structure. It is a hard mineral and only one place lower on Moh's hardness scale than diamond. Gem-quality corundum of all colours (blue, pink, yellow, etc. - except blood-red) is termed sapphire. Blood-red gem-quality corundum is termed as ruby. Different impurities (Cr, V, Fe, etc.) cause different colours in the gemstones.

7. craton

These are the cores of the continents. They are old (at least 500 million years old) and stable sections of crust. They often have very deep "keels" in their centres where the lithosphere becomes very thick. They are mostly composed of crystalline crustal rock such as granite. When two cratons collide and a mountain chain is built, this is an orogen. Examples of important cratons are the Superior and the Churchill that make up most of the Canadian Shield. The Slave craton in the Canadian Arctic is of particular importance as it contains the most significant diamond deposits to date. Diamonds in Russia are found mostly in the Siberian craton. Diamonds in southern African are found mostly in the Kaapvaal and Zimbabwe cratons.

8. diamond

The mineral form of native carbon at high (mantle) pressures and temperatures. It crystallizes in the isometric crystal system. There are two primary growth habits of diamond: octahedra and cubes. Octahedra are dominant. Diamond has a strong octahedral {111} cleavage. Diamond is a pseudormorph of graphite and is the hardest naturally occurring substance known. Although primarily made of carbon, impurities are common and may affect the appearance of the stone. Nitrogen, the most common impurity, can impart a yellow colour, boron in less amounts can cause a blue colour (e.g. the Hope diamond). Hydrogen imparts a purple to gray tinge. Other factors can also cause coloured diamonds. Numerous graphite inclusions cause black diamonds. Deformation of the crystal structure due to stresses in the mantle result in brown, pink, or red diamonds. Breakdown of the crystal (termed becoming metamict) due to radiation damage causes greenish colouration. Coloured diamonds are called fancies.

9. diatreme

This has two meanings. In a structural sense it refers to the inverted conical or "carrot" shaped body typical of many kimberlite pipes. Although the shape can very strongly from the South African model. Fort a la Corne kimberlites are very wide and have a "coupe-style" champagne glass shaped cross-section. Gahcho Kue kimberlites are very irregular. The second meaning is petrological. This refers to the facies of kimberlite in-filling the diatreme. It is often refered to as a tuffisitic breccia. This can be intrepreted as a volcanic rock full of angular xenoliths, mantle and crustal. In the general model of a kimberlite body. diatreme facies kimberlite lies below the crater (pyroclastic) facies kimberlite and above the root zone of hypabyssal kimberlite.

10. eclogite

High grade metamorphic rock. Primarily composed of omphacite and pyrope-almandine-grossular garnet. Kyanite, coesite, corundum, and orthopyroxene are rare accessory minerals.

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