Charlesite Mineral Overview
Mineral name: Charlesite
Mineral class: Sulfate
Known Facts
- Chemical formula: Ca₆(Al,Si)₂(SO₄)₂(OH)₁₂·26H₂O (composition is complex and variable)
- Crystal system: Trigonal
- Mohs hardness: 2.5–3
- Color range: Colorless, white, pale yellow, pale brown
- Luster: Vitreous to silky
- Streak: White
- Cleavage: Poor
- Fracture: Uneven
- Specific gravity: Approximately 1.9–2.1
- Common locations: United States (Massachusetts), United Kingdom, Germany
- Uses: Collector specimens, mineralogical study
- Similar minerals: Ettringite, thaumasite, gypsum
Physical and Optical Characteristics
Charlesite typically forms:
- Acicular (needle-like) crystals
- Radiating sprays or fibrous aggregates
- Crusts lining cavities
Crystals are usually transparent to translucent and often delicate.
Color is generally colorless to white, sometimes with pale yellow or brownish tones. The luster is vitreous on crystal faces and may appear silky in fibrous aggregates.
With a hardness of 2.5 to 3, Charlesite is soft and fragile, requiring careful handling.
Chemical Composition and Structure
Charlesite is a hydrated calcium aluminum sulfate, with a complex and somewhat variable formula:
- Ca₆(Al,Si)₂(SO₄)₂(OH)₁₂·26H₂O
Key components include:
- Calcium (Ca)
- Aluminum (Al), with some silicon (Si) substitution
- Sulfate groups (SO₄²⁻)
- Hydroxyl (OH) groups
- A high water content
Its structure is similar to that of ettringite, consisting of:
- Columnar frameworks of calcium and aluminum polyhedra
- Channels containing sulfate ions and water molecules
The high water content contributes to:
- Low density
- Sensitivity to environmental changes
Formation and Geological Occurrence
Charlesite forms in low-temperature, alkaline environments, particularly:
- Cavities in altered rocks
- Metamorphosed limestone environments
- Hydrothermal alteration zones
It may also form in cement-like conditions, similar to ettringite, where:
- Calcium-, aluminum-, and sulfate-rich solutions interact
It is commonly associated with:
- Ettringite
- Thaumasite
- Gypsum
- Calcite
Common Locations
Notable occurrences of Charlesite include:
- United States: Massachusetts (type locality)
- United Kingdom: Occasional occurrences
- Germany: Additional localities
It is a rare mineral, typically found in small quantities.
Uses and Practical Significance
Charlesite has no industrial uses. Its importance lies in:
- Mineral collecting: Especially for collectors of rare sulfates
- Scientific study: Understanding hydrated sulfate mineral systems
Its delicate crystal habit makes it mainly a specimen mineral.
Similar and Related Minerals
Charlesite may be confused with:
- Ettringite: Very similar structure and habit
- Thaumasite: Also fibrous and hydrated but contains carbonate
- Gypsum: Softer and typically forms larger crystals
Distinguishing Charlesite often requires:
- Chemical analysis
- Careful observation of associations and environment
Identification Notes for Collectors
- Look for white, needle-like crystal sprays in cavities
- Note association with ettringite and gypsum
- Check for softness and low density
- Observe silky luster in fibrous aggregates
- Use analytical methods for confirmation due to similarity with related sulfates
Charlesite is a rare hydrated sulfate mineral, recognized for its acicular crystal habit and similarity to ettringite, and is primarily of interest to collectors and mineralogists studying low-temperature sulfate systems.