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Andradite is a member of the garnet group, specifically the ugrandite series (calcium-rich garnets). It is a nesosilicate mineral with a distinctive chemical composition of Ca3Fe2(SiO4)3. It exhibits a wide range of colors, from yellow-green (Demantoid) to black (Melanite), and is known for its high dispersion, particularly in the Demantoid variety, which gives it a fiery brilliance. It crystallizes in the isometric system, typically forming dodecahedral or trapezohedral crystals.
How to Identify
- Color
- Variable, including green (Demantoid), yellow (Topazolite), brown, black (Melanite), and reddish-brown. The green color of Demantoid is due to trace amounts of chromium and/or iron.
- Luster
- Vitreous to adamantine, particularly in Demantoid, which can exhibit a diamond-like luster.
- Texture
- Typically granular or massive, but well-formed crystals are common.
- Crystal Form
- Commonly dodecahedral (12-sided) or trapezohedral (24-sided) crystals, or a combination of both. Crystals are often euhedral to subhedral.
- Cleavage
- None. Garnets are known for their lack of cleavage, exhibiting conchoidal to uneven fracture.
- Geological Environment
- Contact metamorphic aureoles, skarns, serpentinites, alkaline igneous rocks, hydrothermally altered rocks. Often associated with minerals like epidote, chlorite, calcite, and magnetite.
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 3.85-3.99 g/cm³
- Crystal System: Isometric
- Color: Green (Demantoid), yellow (Topazolite), brown, black (Melanite), reddish-brown
- Luster: Vitreous to adamantine
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Calcium iron silicate, Ca3Fe2(SiO4)3
Quick Check
- Color: Green, yellow, brown, black, reddish-brown
- Luster: Vitreous to adamantine
- Streak: White
Physical Characteristics
- Crystal Habit: Typically dodecahedral, trapezohedral, or a combination of both; also massive or granular.
- Cleavage Type: Absent
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to adamantine
Formation
Andradite is a calcium-iron garnet that typically forms in contact metamorphic aureoles within impure limestones or calc-silicate rocks, skarns, and serpentinites. It can also be found in some igneous rocks, particularly alkaline igneous rocks, and in hydrothermally altered rocks. Its formation is favored by high temperatures and variable pressures, often involving metasomatic fluid interaction.
Usage
Gemstone (especially Demantoid), abrasive (less common than other garnets due to its relative rarity and value), mineralogical specimens.
Age Distribution
Andradite forms in a variety of geological settings and is not restricted to a specific geological age. Its formation is tied to specific metamorphic and metasomatic processes that can occur throughout Earth's history.
Where to Find
Ural Mountains, Russia
Famous for producing high-quality Demantoid garnets, particularly from the Bobrovka River area.
Val Malenco, Italy
Known for black Melanite garnets and some green Demantoid.
Namibia
Significant source of Demantoid garnets, often with a more yellowish-green hue than Russian material.
Iran
Produces Demantoid garnets, sometimes with a distinct yellowish-green color.
Arizona, USA
Localities for Melanite and other varieties of Andradite.
Mexico
Known for Topazolite (yellow Andradite) and other varieties.
Finding Tips
Geological Context
Focus on areas with contact metamorphic rocks, especially skarns formed from the alteration of impure limestones or dolomites by igneous intrusions. Serpentinites and alkaline igneous rocks are also potential hosts.
Associated Minerals
Look for Andradite in association with minerals such as epidote, chlorite, calcite, magnetite, vesuvianite, and diopside, which are common in its formation environments.
Crystal Habit
Andradite often forms well-developed dodecahedral or trapezohedral crystals. Identifying these characteristic crystal forms can aid in identification in the field.
Luster and Color
The vitreous to adamantine luster, especially of Demantoid, can be a key indicator. The range of colors, particularly the vibrant green of Demantoid, is also distinctive.
Hardness
Andradite's hardness of 6.5-7 on the Mohs scale means it will scratch glass and steel, which can be a useful field test.
Similar Rocks
Grossular Garnet
Grossular
Also known as: Hessonite, Tsavorite
Almandine Garnet
Almandine
Also known as: Almandite
Spessartine Garnet
Spessartine
Also known as: Spessartite
Pyrope Garnet
Pyrope
Also known as: Pyrope
Scientific Classification
- Mineral Class
- Nesosilicate
- Group
- Garnet Group (Ugrandite Series)
- Crystal System
- Isometric (Cubic)
- Chemical Formula
- Ca3Fe2(SiO4)3
- Composition
- Calcium iron silicate
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