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Grossular is a calcium-aluminum garnet mineral, a member of the ugrandite series (Uvarovite-Grossular-Andradite). Its chemical formula is Ca3Al2(SiO4)3. It exhibits a wide range of colors, including colorless, white, yellow, orange, reddish-orange (Hessonite), green (Tsavorite, Hydrogrossular), and brown. The color variations are due to trace elements; for example, chromium and vanadium cause the vibrant green of Tsavorite. When found 'in matrix,' it means the grossular crystals are embedded within the host rock in which they formed, providing context to their geological origin and often enhancing their aesthetic appeal for collectors. The matrix can be various calc-silicate minerals, marble, or skarn minerals.
How to Identify
- Color
- Highly variable: colorless, white, yellow, orange, reddish-orange (Hessonite), green (Tsavorite, Hydrogrossular), brown. The specific color can be a key indicator of variety.
- Luster
- Vitreous (glassy) to resinous. Tsavorite often has a particularly brilliant vitreous luster.
- Texture
- Typically forms euhedral to subhedral dodecahedral or trapezohedral crystals. When in matrix, the crystals will be embedded in the host rock, which can be fine-grained to coarse-grained depending on the matrix mineralogy.
- Crystal Form
- Most commonly dodecahedral (12-sided) or trapezohedral (24-sided) crystals. Combinations of these forms are also common. Crystals are typically isometric.
- Cleavage
- None. Garnets are known for their lack of cleavage, which is a key diagnostic feature.
- Geological Environment
- Metamorphic rocks (especially calc-silicate rocks, marbles, skarns), contact metamorphic aureoles around igneous intrusions, serpentinites (for some hydrogrossular). Associated minerals often include diopside, wollastonite, calcite, quartz, epidote, and vesuvianite.
Key Facts
- Hardness: 6.5-7.5 on Mohs scale
- Specific Gravity: 3.57-3.73 g/cm³ (can be lower for hydrogrossular, 3.3-3.5 g/cm³)
- Crystal System: Isometric (Cubic)
- Color: Colorless, white, yellow, orange, reddish-orange, green, brown
- Luster: Vitreous to resinous
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Calcium aluminum silicate (Ca3Al2(SiO4)3)
Quick Check
- Color: Variable (colorless, yellow, orange, green, brown)
- Luster: Vitreous to resinous
- Streak: White
Physical Characteristics
- Crystal Habit: Typically dodecahedral, trapezohedral, or combinations of these forms; granular, massive.
- Cleavage Type: None (parting may be observed in some cases, but not true cleavage)
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to resinous
Formation
Grossular garnet typically forms during regional or contact metamorphism of impure calcareous rocks (limestones, marls) or calc-silicate rocks. It can also form in skarns, which are metasomatic rocks formed at the contact between an igneous intrusion and carbonate rocks. The presence of calcium, aluminum, and silicon, along with appropriate temperature and pressure conditions, are crucial for its formation. Hydrogrossular, a hydrous variety, forms under lower temperature and higher water activity conditions.
Usage
Gemstone (especially Tsavorite and Hessonite varieties), abrasive (less common than almandine or pyrope), mineralogical specimens for collectors, scientific research into metamorphic processes and skarn formation.
Age Distribution
Grossular garnet forms in a wide range of geological ages, from Precambrian to Cenozoic, depending on the specific metamorphic or metasomatic events.
Where to Find
Kenya and Tanzania
World-renowned for high-quality Tsavorite (green grossular) found in Neoproterozoic metamorphic rocks of the Mozambique Belt.
Canada (Quebec, British Columbia)
Known for various colors of grossular, including orange-brown Hessonite and green varieties, often in skarn deposits.
Mexico (Sierra de Cruces, Coahuila)
Produces fine orange-brown Hessonite crystals, often in skarn environments.
USA (Vermont, California, Alaska)
Vermont is known for green grossular (hydrogrossular) in serpentinites. California and Alaska have occurrences in metamorphic and skarn settings.
Italy (Val d'Ala, Piedmont)
Classic locality for well-formed grossular crystals, often in calc-silicate rocks.
Pakistan
Produces fine quality Hessonite and other grossular varieties.
Finding Tips
Target Metamorphic and Skarn Environments
Focus your search on areas known for contact metamorphic aureoles around granitic intrusions into carbonate rocks, or regionally metamorphosed calc-silicate units. Look for outcrops with other calc-silicate minerals like diopside, wollastonite, and epidote.
Look for Characteristic Crystal Forms
Grossular often forms distinct dodecahedral or trapezohedral crystals. Even if embedded, these forms can be discernible on weathered surfaces or in broken rock.
Observe Color and Luster
The wide range of colors, from colorless to vibrant green or orange, combined with a vitreous to resinous luster, can help distinguish grossular from other minerals in the matrix.
Test Hardness
With a Mohs hardness of 6.5-7.5, grossular will scratch glass and steel. This helps differentiate it from softer associated minerals.
Check for Lack of Cleavage
Garnets do not exhibit cleavage. If you observe distinct cleavage planes, it is likely not grossular.
Similar Rocks
Andradite Garnet
Andradite (Ca3Fe2(SiO4)3)
Also known as: Demantoid (green variety), Melanite (black variety), Topazolite (yellow variety)
Uvarovite Garnet
Uvarovite (Ca3Cr2(SiO4)3)
Also known as: Chromium Garnet
Vesuvianite
Vesuvianite (Ca19(Al,Mg,Fe)13(SiO4)10(Si2O7)4(OH,F,O)10)
Also known as: Idocrase
Epidote
Epidote (Ca2(Al,Fe)3(SiO4)3(OH))
Also known as: Pistacite
Scientific Classification
- Mineral Class
- Silicate
- Group
- Garnet Group (Ugrandite Series)
- Crystal System
- Isometric
- Chemical Formula
- Ca3Al2(SiO4)3
- Composition
- Calcium aluminum silicate
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