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Grossular is a calcium-aluminum garnet mineral, belonging to the ugrandite series (Uvarovite-Grossular-Andradite). Its chemical formula is Ca3Al2(SiO4)3. The name 'grossular' comes from the botanical name for gooseberry, 'Grossularia', referring to the typical greenish color of some varieties. However, grossular exhibits a wide range of colors, including colorless, white, yellow, orange, reddish-orange (hessonite), brown, and various shades of green (tsavorite). It crystallizes in the isometric system, commonly forming dodecahedral or trapezohedral crystals, or combinations thereof. It is a relatively hard and dense mineral, often found in metamorphic and metasomatic rocks.
How to Identify
- Color
- Grossular garnet can be colorless, white, yellow, orange, reddish-orange (hessonite), brown, light green, or emerald green (tsavorite). The color is a key indicator, especially for the gem varieties.
- Luster
- Vitreous to resinous.
- Texture
- Typically granular or massive, but well-formed crystals exhibit smooth, often shiny faces.
- Crystal Form
- Commonly dodecahedral (12-sided) or trapezohedral (24-sided) crystals, or combinations of these forms. Can also be anhedral (without well-formed faces) in massive aggregates.
- Cleavage
- None. Garnets are known for their lack of cleavage, exhibiting conchoidal to subconchoidal fracture.
- Geological Environment
- Look for grossular in metamorphosed impure limestones, dolomites, marls, and skarns. It is often associated with minerals like diopside, wollastonite, vesuvianite, epidote, and calcite.
Key Facts
- Hardness: 6.5-7.5 on the Mohs scale
- Specific Gravity: 3.57-3.73 g/cm³
- Crystal System: Isometric
- Color: Colorless, white, yellow, orange, reddish-orange, brown, light green, emerald green
- Luster: Vitreous to resinous
- Transparency: Transparent to translucent
- Fracture: Conchoidal to subconchoidal
- Cleavage: None
- Composition: Calcium aluminum silicate
Quick Check
- Color: Variable (colorless, white, yellow, orange, brown, green)
- Luster: Vitreous to resinous
- Streak: White
Physical Characteristics
- Crystal Habit: Typically dodecahedral, trapezohedral, or combinations; also massive or granular.
- Cleavage Type: Absent
- Fracture Type: Conchoidal to subconchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to resinous
Formation
Grossular garnet typically forms during regional or contact metamorphism of impure calcareous rocks (limestones or dolomites) or marls. 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. Temperatures generally range from 400°C to 700°C, and pressures can vary widely depending on the metamorphic environment.
Usage
Grossular garnet is primarily used as a gemstone, particularly the green tsavorite and orange-brown hessonite varieties. Its hardness and attractive colors make it suitable for jewelry. Less commonly, it can be used as an abrasive, though other garnet species are more frequently employed for this purpose. Scientific research also utilizes grossular for understanding metamorphic processes and conditions.
Age Distribution
Grossular garnet forms in a variety of geological settings and thus can be found in rocks ranging from Precambrian to Cenozoic in age, depending on the specific metamorphic or metasomatic event.
Where to Find
Kenya and Tanzania
These East African countries are famous for producing high-quality tsavorite (green grossular), particularly in the Taita-Taveta region of Kenya and the Arusha region of Tanzania.
Canada
Quebec and British Columbia have localities known for producing grossular, including some fine hessonite and green varieties.
Mexico
Mexico is a notable source of orange to yellow grossular, often found in skarn deposits.
United States
Localities in Vermont, California, and Alaska have produced grossular garnet, including some gem-quality material.
Italy
The Val d'Ala in Piedmont is a classic locality for grossular, often found in serpentinites.
Sri Lanka
Known for producing hessonite (orange-brown grossular) from alluvial deposits.
Finding Tips
Target Metamorphic Terrains
Focus your search on areas with known occurrences of regionally or contact metamorphosed calcareous rocks, such as marble, calc-silicate rocks, and skarns.
Look for Associated Minerals
Grossular often co-occurs with other calc-silicate minerals like diopside, wollastonite, vesuvianite, epidote, and calcite. The presence of these minerals can indicate a favorable environment.
Examine Outcrops and Stream Beds
In metamorphic terrains, examine rock outcrops for characteristic crystal forms. In alluvial environments, grossular's hardness and density mean it can be found in stream gravels and placers.
Check for Crystal Habits
Keep an eye out for the distinctive dodecahedral or trapezohedral crystal forms, even if they are small or embedded in the matrix.
Color Variation
Be aware of the wide range of colors grossular can exhibit. Don't limit your search to just green or orange; colorless, yellow, and brown varieties are also common.
Similar Rocks
Andradite Garnet
Ca3Fe2(SiO4)3
Also known as: Demantoid (green variety), Melanite (black variety), Topazolite (yellow variety)
Pyrope Garnet
Mg3Al2(SiO4)3
Also known as: Rhodolite (purple-red variety)
Almandine Garnet
Fe3Al2(SiO4)3
Also known as: Common Garnet
Spessartine Garnet
Mn3Al2(SiO4)3
Also known as: Mandarin Garnet (orange variety)
Scientific Classification
- Mineral Class
- Silicates
- Group
- Garnet Group (Ugrandite Series)
- Crystal System
- Isometric
- Chemical Formula
- Ca3Al2(SiO4)3
- Composition
- Calcium aluminum silicate
Explore Grossular Garnet
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