Knorringite
Mg3Cr2(SiO4)3
A chromium-rich magnesium garnet of the pyrope series that crystallizes in the deep mantle and is a valuable diamond indicator mineral.
- Mohs hardness
- 7-7.5
- Color
- green to bluish green
- Type
- mineral
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Overview
Knorringite is the chromium-dominant magnesian end-member of the garnet group, structurally part of the pyrope series. Chromium (Cr3+) in the octahedral site gives it a green to bluish-green color and ties it geochemically to high-pressure, chromium-rich mantle rocks.
Pure knorringite is rare; most natural material is high-chromium pyrope (often called chrome pyrope) trending toward the knorringite composition. It is isometric (cubic) and hard, like other garnets.
The mineral is best known to diamond explorers, because chromium-rich pyrope-knorringite garnets are classic indicator minerals carried up from the diamond stability field.
Formation & geology
Knorringite forms at very high pressures in the upper mantle, within chromium-bearing peridotites and other ultramafic rocks. It requires both high pressure and a chromium-rich environment, conditions met deep beneath stable cratons.
Kimberlite and lamproite magmas sample these mantle rocks and transport knorringite-bearing garnets to the surface as xenocrysts. It is therefore found in kimberlite pipes and in the eluvial and alluvial deposits derived from them.
Classic occurrences include the kimberlites of southern Africa (where it was first described from Kao, Lesotho) and Siberia, both major diamond provinces.
How to identify it
Look for green to bluish-green, hard garnet grains recovered from kimberlite or diamond-indicator concentrate. Mohs hardness is about 7-7.5 with a white streak and vitreous luster.
It resembles other green chromium garnets such as uvarovite and chrome pyrope, and distinguishing true knorringite requires microprobe analysis showing high chromium and magnesium. Geological setting is diagnostic: a chrome-rich green pyrope garnet from kimberlite or mantle peridotite points toward the knorringite component, unlike crustal uvarovite from chromite seams.
Uses & significance
Knorringite's chief importance is exploration: chromium-rich pyrope-knorringite garnets are key diamond indicator minerals, and their chemistry helps geologists assess whether a kimberlite tapped the diamond stability field.
It is not used as a gemstone because suitable crystals are scarce, though its bright chrome-green relatives are admired by collectors. It has no established metaphysical tradition distinct from other pyrope garnets.
Frequently asked questions
Why do diamond explorers care about knorringite?
Chromium-rich pyrope-knorringite garnets form at the same high pressures as diamonds, so finding them suggests a kimberlite sampled the diamond stability field.
What gives knorringite its color?
Chromium in the garnet structure produces its green to bluish-green hue.
Is pure knorringite common?
No. Most natural material is high-chromium pyrope trending toward knorringite rather than the pure end-member.
Where is knorringite found?
In mantle peridotites and the kimberlites that carry them upward, especially in the diamond fields of southern Africa and Siberia.
Knorringite guides
In-depth guides for identifying, valuing, and understanding Knorringite.
