Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Diopside in calcite refers to a rock or mineral specimen where crystals of the pyroxene mineral diopside (CaMgSi2O6) are embedded within a matrix of the carbonate mineral calcite (CaCO3). Diopside typically appears as green, greenish-black, or white prismatic crystals, while calcite is usually white, gray, or colorless and exhibits its characteristic rhombohedral cleavage. The combination often results from the metamorphism of siliceous dolomitic limestones, where the silica reacts with the dolomite to form diopside, leaving calcite as the dominant matrix mineral.
How to Identify
- Color
- Diopside: typically light to dark green, greenish-black, white, or rarely colorless. Calcite: usually white, gray, colorless, or pale yellow.
- Luster
- Diopside: Vitreous to dull. Calcite: Vitreous to sub-vitreous, sometimes pearly on cleavage surfaces.
- Texture
- Granular to crystalline, with diopside crystals (often prismatic) embedded within a finer-grained or coarsely crystalline calcite matrix.
- Crystal Form
- Diopside: Prismatic crystals, often with a square or octagonal cross-section, sometimes columnar or granular. Calcite: Rhombohedral crystals, granular aggregates, massive.
- Cleavage
- Diopside: Two distinct cleavages at approximately 87° and 93° (pyroxene cleavage). Calcite: Perfect rhombohedral cleavage in three directions, producing rhomb-shaped fragments.
- Geological Environment
- Contact metamorphic aureoles (skarns) around igneous intrusions, regionally metamorphosed carbonate sequences, and hydrothermal veins associated with these environments.
Key Facts
- Hardness: Diopside: 5.5-6.5 (Mohs). Calcite: 3 (Mohs).
- Specific Gravity: Diopside: 3.22-3.38. Calcite: 2.71.
- Crystal System: Diopside: Monoclinic. Calcite: Trigonal.
- Color: Diopside: Green, white, gray, black. Calcite: White, colorless, gray, yellow.
- Luster: Diopside: Vitreous to dull. Calcite: Vitreous to sub-vitreous.
- Transparency: Diopside: Transparent to translucent. Calcite: Transparent to opaque.
- Fracture: Diopside: Uneven to conchoidal. Calcite: Conchoidal to uneven.
- Cleavage: Diopside: Two distinct cleavages at 87° and 93°. Calcite: Perfect rhombohedral cleavage in three directions.
- Composition: Diopside: Calcium Magnesium Silicate (CaMgSi2O6). Calcite: Calcium Carbonate (CaCO3).
Quick Check
- Color: Green (diopside) in white/gray (calcite)
- Luster: Vitreous to dull (diopside), Vitreous (calcite)
- Streak: White (both diopside and calcite)
Physical Characteristics
- Crystal Habit: Diopside: Prismatic, columnar, granular. Calcite: Rhombohedral, massive, granular.
- Cleavage Type: Diopside: Prismatic {110}. Calcite: Rhombohedral {1011}.
- Fracture Type: Diopside: Uneven to conchoidal. Calcite: Conchoidal to uneven.
- Tenacity: Diopside: Brittle. Calcite: Brittle.
- Luster Type: Diopside: Vitreous to dull. Calcite: Vitreous to sub-vitreous, sometimes pearly.
Formation
Diopside in calcite typically forms during contact or regional metamorphism of impure limestones or dolomites. The presence of silica (SiO2) and magnesium (Mg) in the original carbonate rock, along with calcium (Ca), under elevated temperatures and pressures, leads to the reaction that forms diopside. This often occurs in skarn deposits where carbonate rocks are intruded by igneous bodies, or in regionally metamorphosed carbonate sequences. The diopside crystals grow within the calcite matrix.
Usage
Primarily of interest to mineral collectors, geologists, and researchers studying metamorphic processes. Large, clear diopside crystals within calcite can be cut as cabochons or faceted for jewelry, though this is less common for the 'in calcite' variety. Calcite itself is used in construction, agriculture, and as a flux in metallurgy.
Age Distribution
Variable, depending on the age of the protolith and the metamorphic event. Can range from Precambrian to Cenozoic.
Where to Find
Adirondack Mountains, New York, USA
Known for various metamorphic minerals, including diopside in marble.
Ontario and Quebec, Canada
Many localities in the Grenville Province yield excellent diopside crystals in calcite.
Italy (e.g., Val Malenco)
Classic localities for diopside, often found in metamorphic rocks.
Myanmar (Burma)
Source of gem-quality diopside, sometimes found in association with calcite.
Pakistan
Various metamorphic terrains yield diopside, including in carbonate matrices.
Finding Tips
Look for Metamorphic Terrains
Focus on areas known for contact or regional metamorphism of carbonate rocks, especially near igneous intrusions.
Identify Calcite Matrix
Calcite will effervesce vigorously with dilute hydrochloric acid (HCl). Look for its characteristic rhombohedral cleavage.
Spot Diopside Crystals
Search for green, prismatic crystals embedded within the calcite. The two distinct cleavages at nearly 90 degrees are a key identifier for diopside.
Check for Skarn Indicators
Skarn deposits often contain other calc-silicate minerals like garnet, epidote, and tremolite, which can indicate a favorable environment for diopside.
Similar Rocks
Tremolite in Calcite
Ca2Mg5Si8O22(OH)2 in CaCO3
Also known as: Tremolite Marble
Wollastonite in Calcite
CaSiO3 in CaCO3
Also known as: Wollastonite Skarn
Forsterite in Calcite
Mg2SiO4 in CaCO3
Also known as: Forsterite Marble
Scientific Classification
- Mineral Class
- Diopside: Inosilicate (Pyroxene Group). Calcite: Carbonate.
- Group
- Diopside: Pyroxene Group. Calcite: Calcite Group.
- Crystal System
- Diopside: Monoclinic. Calcite: Trigonal.
- Chemical Formula
- Diopside: CaMgSi2O6. Calcite: CaCO3.
- Composition
- Diopside: Calcium magnesium silicate. Calcite: Calcium carbonate.
Explore Diopside in Calcite
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.