Hexagonite Mineral Overview
- Mineral name: Hexagonite
- Mineral class: Silicate (inosilicate, amphibole group – tremolite variety)
- Chemical formula: Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
- Crystal system: Monoclinic (despite name)
- Mohs hardness: 5–6
- Color range: Purple, lavender, lilac, pink
- Luster: Vitreous to silky
- Streak: White
- Cleavage: Perfect (two directions at ~56° and 124°)
- Fracture: Splintery
- Specific gravity: Approximately 2.9–3.1
- Transparency: Transparent to translucent
- Common locations: United States, Canada
- Uses: Gemstone, collector mineral
- Similar minerals: Tremolite, lepidolite, kunzite
Naming and Classification
Hexagonite is a manganese-rich variety of tremolite, which belongs to the amphibole group.
Despite its name, it is not hexagonal. The name comes from:
- Its apparent hexagonal crystal shape, caused by twinning
It is chemically similar to tremolite but distinguished by:
- Manganese (Mn) content, which gives it its color
Physical and Optical Characteristics
Hexagonite typically occurs as:
- Elongated prismatic crystals
- Fibrous or columnar aggregates
Its color is its most distinctive feature:
- Lavender
- Lilac
- Purple to pink tones
The luster ranges from vitreous to silky, especially in fibrous forms.
With a Mohs hardness of 5–6, it is moderately hard.
Color (Key Feature)
Hexagonite is notable for:
- Soft purple to lilac coloration
This color is caused by:
- Manganese (Mn) in the crystal structure
It is one of the few naturally occurring purple amphibole minerals.
Chemical Composition and Structure
Hexagonite is composed of:
- Calcium (Ca)
- Magnesium (Mg)
- Iron (Fe)
- Manganese (Mn)
- Silicon (Si)
- Oxygen (O)
- Hydroxyl (OH)
It has a double-chain inosilicate structure, typical of amphiboles, forming:
- Long, fibrous or prismatic crystals
Formation and Geological Occurrence
Hexagonite forms in metamorphic environments, particularly:
Metamorphosed Carbonate Rocks
- Limestone or dolomite subjected to heat and pressure
Typical formation conditions include:
- Moderate to high temperatures
- Presence of manganese and calcium-rich fluids
It is commonly associated with:
- Calcite
- Tremolite
- Other amphiboles
Common Locations
Hexagonite is relatively rare and found in:
- United States: New York (notable occurrences)
- Canada: Quebec
These localities are known for amphibole minerals in metamorphic rocks.
Uses and Practical Significance
Hexagonite is valued for:
Gemstone Use
- Cut into cabochons
- Occasionally faceted
Collector Mineral
- Unusual purple color
- Interest among mineral collectors
It has no industrial use.
Similar and Related Minerals
Hexagonite can resemble:
- Tremolite: Usually white or green
- Lepidolite: Softer and flaky
- Kunzite: Harder and more transparent
Color and crystal structure help distinguish hexagonite.
Identification Notes for Collectors
- Look for lavender to purple prismatic or fibrous crystals
- Note moderate hardness (5–6)
- Observe silky to vitreous luster
- Check amphibole cleavage angles (~56° and 124°)
- Consider association with metamorphic carbonate rocks
Hexagonite is a manganese-rich variety of tremolite, recognized for its distinctive purple coloration and amphibole structure, making it a rare and attractive mineral for collectors and occasional gemstone use.