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Diopside is a monoclinic pyroxene mineral with the chemical formula CaMgSi2O6. When found 'in matrix', it refers to diopside crystals or grains embedded within the host rock from which it formed. The matrix can vary widely depending on the geological environment, including marble, schist, gneiss, or various igneous rocks. Diopside is an end-member of the clinopyroxene solid solution series, forming a complete solid solution with hedenbergite (CaFeSi2O6) and an incomplete solid solution with augite. Its color can range from colorless to white, light green, dark green, brown, or black, often influenced by trace elements. Chromian diopside, a vibrant green variety, is particularly prized as a gemstone.
How to Identify
- Color
- Typically light to dark green, but can be colorless, white, gray, brown, or black. Chromian diopside is emerald green.
- Luster
- Vitreous to dull.
- Texture
- Crystals are typically prismatic, often with a square or octagonal cross-section. Can be granular or massive within the matrix.
- Crystal Form
- Monoclinic system, typically forming short to long prismatic crystals with characteristic pyroxene cleavage angles (approximately 87° and 93°).
- Cleavage
- Two distinct cleavages intersecting at nearly 90 degrees (specifically 87° and 93°), characteristic of pyroxenes.
- Geological Environment
- Metamorphic rocks (marbles, skarns, calc-silicate gneisses), mafic and ultramafic igneous rocks (basalts, gabbros, peridotites, kimberlites).
Key Facts
- Hardness: 5-6 (Mohs scale)
- Specific Gravity: 3.22-3.38
- Crystal System: Monoclinic
- Color: Green, white, gray, brown, black, colorless
- Luster: Vitreous to dull
- Transparency: Transparent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect on {110} at 87° and 93°
- Composition: Calcium Magnesium Silicate (CaMgSi2O6)
Quick Check
- Color: Green (most common), white, gray, brown, black, colorless
- Luster: Vitreous to dull
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, columnar, granular, massive
- Cleavage Type: Perfect in two directions, nearly at right angles (87° and 93°)
- Fracture Type: Uneven to conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to dull
Formation
Diopside is a common mineral in metamorphic rocks, particularly those formed from the metamorphism of siliceous dolomites and limestones (calc-silicate rocks, skarns). It also occurs in mafic and ultramafic igneous rocks (e.g., peridotite, pyroxenite, basalt, gabbro) and kimberlites. Its formation is favored by high temperatures and moderate to high pressures.
Usage
Gemstone (especially the chromian variety), mineral specimens for collectors, scientific research (as an indicator of metamorphic and igneous conditions), and occasionally as a component in ceramics or refractories.
Age Distribution
Diopside forms in a wide range of geological settings and thus can be found in rocks of various ages, from Precambrian to Cenozoic, depending on the specific geological event.
Where to Find
Italy
Val di Fassa, Trentino-Alto Adige (classic locality for well-formed crystals).
Myanmar (Burma)
Mogok region (known for gem-quality chromian diopside).
Russia
Aldan Shield, Yakutia (source of star diopside and chromian diopside).
Canada
Quebec and Ontario (found in metamorphic terrains).
USA
New York, California, and various other states in metamorphic and igneous settings.
Pakistan
Northern Areas (gem-quality diopside).
Finding Tips
Geological Context
Look for diopside in areas known for contact metamorphism of carbonate rocks (skarns) or regional metamorphism of impure limestones and dolomites. Also check mafic and ultramafic igneous intrusions.
Associated Minerals
Diopside often occurs with calcite, dolomite, tremolite, grossular garnet, vesuvianite, wollastonite, forsterite, and various amphiboles.
Crystal Habit
Identify its characteristic prismatic crystals with nearly 90-degree cleavage. The green color, especially the vivid green of chromian diopside, is a strong indicator.
Hardness Test
Its hardness of 5-6 on the Mohs scale means it can scratch glass but can be scratched by quartz.
Similar Rocks
Augite
Complex (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
Also known as: Common Pyroxene
Hedenbergite
CaFeSi2O6
Also known as: Iron Diopside
Tremolite
Ca2Mg5Si8O22(OH)2
Also known as: Calcium Magnesium Silicate Hydroxide
Epidote
Ca2(Al,Fe)3(SiO4)3(OH)
Also known as: Pistacite
Scientific Classification
- Mineral Class
- Silicates
- Group
- Pyroxene Group (Clinopyroxene Subgroup)
- Crystal System
- Monoclinic
- Chemical Formula
- CaMgSi2O6
- Composition
- Calcium Magnesium Silicate
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