Stevensite is a rare phyllosilicate mineral belonging to the smectite group of clay minerals. It is a hydrated sodium‑magnesium silicate that typically occurs as fine‑grained, earthy to lamellar aggregates.
Chemical composition
The idealized chemical formula is commonly expressed as Na₀.₃₃(Mg,Fe)₃Si₄O₁₀(OH)₂·nH₂O, indicating variable sodium content, substitution of iron for magnesium, and variable amounts of interlayer water. The exact stoichiometry may vary depending on the locality and formation conditions.
Crystallography
Stevensite crystallizes in the monoclinic crystal system and exhibits a layered sheet structure characteristic of smectite minerals. Its crystallographic parameters are not uniformly defined due to the mineral’s structural variability.
Physical properties
| Property | Typical range / description |
|---|---|
| Color | White, gray, or colorless |
| Habit | Massive, earthy, lamellar, or as fine aggregates |
| Mohs hardness | 1–2 |
| Luster | Dull to earthy |
| Streak | White |
| Specific gravity | ~2.4–2.6 |
| Transparency | Translucent to opaque |
| Optical properties | Biaxial (+); exhibits strong pleochroism in thin sections |
Occurrence and formation
Stevensite forms through the low‑temperature alteration of volcanic glass and tephra, particularly in marine and lacustrine sedimentary environments. It has been reported from a limited number of localities, including volcanic ash deposits in the Philippines, Italy, and the United States (e.g., the Coso Range, California). The mineral commonly co‑occurs with other smectite‑group clays such as montmorillonite and nontronite.
Uses
Due to its rarity, stevensite has no significant industrial applications and is of interest primarily to mineralogists and collectors. Its adsorption properties are similar to other smectite clays, but its scarcity precludes commercial use.
Etymology
The mineral was named in honor of an individual bearing the surname Stevens, recognized for contributions to geology or mineralogy. Specific details of the namesake’s identity are not uniformly recorded in the literature.
History of identification
Stevensite was first described in the early 20th century following petrographic study of altered volcanic deposits. The International Mineralogical Association (IMA) subsequently approved the name and assigned the mineral symbol “Stv”.
References
- Mindat.org – Stevensite mineral data.
- Handbook of Mineralogy – Stevensite entry.
- International Mineralogical Association – List of approved mineral symbols.