Eugsterite
Chemical formula: Na<sub>4</sub>Ca(S<sup>6+</sup>O<sub>4</sub>)<sub>3</sub>·2H<sub>2</sub>O
A brittle, water-soluble sodium and calcium sulfate, forming acicular crystals and efflorescences in dried salt lakes.
Description
## Characteristics Eugsterite is a hydrated sodium and calcium sulfate, belonging to the evaporite mineral group. It most commonly occurs as brittle, white or colorless efflorescences and crusts. It forms very small, bladed or acicular crystals, which often arrange into radial or tangled aggregates. Due to its softness and solubility, it is a very delicate mineral. ## Physical Properties It is one of the softer minerals, with a Mohs hardness of only 1-2, meaning it can be scratched with a fingernail. The luster on crystal surfaces is vitreous, and on cleavage planes it can be pearly; in earthy aggregates, it is dull. Crystals can be transparent to translucent. Its density is approximately 2.55 g/cm³. ## Colors and Varieties Eugsterite is usually colorless or white. No colored varieties or trade names are distinguished. ## History and Name The mineral was described in 1980 by F. A. Blair, A. Pabst, and D. L. Sawyer. Its name honors Hans P. Eugster (1925-1982), a Swiss-American geochemist from Johns Hopkins University, for his fundamental contributions to the study of the genesis of salt deposits and evaporite minerals. ## Uses Eugsterite has no industrial significance. It is an object of scientific interest and is sought after by collectors specializing in rare minerals or evaporite minerals.
Diagnostic features
## Identification Key diagnostic features of eugsterite include its extreme softness (hardness 1-2), characteristic occurrence environment (dried salt lakes), and most importantly, complete solubility in water. In the field, it occurs as white, powdery or fibrous efflorescences. ## Distinguishing from Similar Minerals Eugsterite can be confused with other white, soft evaporite sulfates. - **Gypsum:** It is also soft (hardness 2), but has different crystal forms and is much less soluble in water. - **Thenardite:** Often co-occurs with eugsterite, but crystallizes in the orthorhombic system and has different optical properties. - **Mirabilite:** It is very similar in terms of solubility and appearance, but is unstable in dry air and rapidly effloresces to thenardite. ## Crystal Forms It forms small, elongated crystals with a bladed or acicular habit. They usually occur as radial or haphazardly tangled aggregates, as well as delicate, powdery or earthy efflorescences and crusts.
Geological environment
## Genesis Eugsterite is a mineral of evaporitic origin. It forms as a result of water evaporation in endorheic, saline lakes (so-called playas) in arid and semi-arid climates. It can also form as a secondary efflorescence on the surface of saline soils. ## Mineral Associations It most commonly co-occurs with other evaporite minerals, such as thenardite, bloedite, gypsum, halite, mirabilite, and glauberite. ## Localities The most important eugsterite localities worldwide are primarily locations in the United States, including the type locality – the bottom of the dried Lake Lucero in New Mexico and Saline Valley in California. It is also found in salt lakes (salars) in Chile (e.g., Salar de Atacama) and Argentina, as well as in volcanic sediments on Deception Island (Antarctica).
Rarity
Not very common
Collector aspects
## Quality Criteria Eugsterite specimens are not a typical collector's item due to their instability and inconspicuous appearance. For specialized collectors, the most valuable samples are those with well-formed, sharp microcrystals forming aesthetic aggregates, preferably accompanied by other rare evaporites. Purity and lack of mechanical damage are crucial. ## Popular Localities The most known and valued specimens by collectors, often serving as reference material, come from classic US localities such as Lake Lucero in New Mexico and Saline Valley in California.
Care and storage
## Cleaning Eugsterite is extremely sensitive to water, in which it rapidly dissolves. Cleaning must be done exclusively dry. To remove dust, a very soft brush can be used, or dust can be carefully blown off with compressed air from a safe distance. ## What to Avoid Contact with water and all other liquids must be absolutely avoided. The mineral is also hygroscopic, meaning it can absorb moisture from the air, leading to its slow degradation. It should be protected from humid environments, sudden temperature changes, and chemical contamination. ## Storage Eugsterite specimens require storage in dry conditions. It is best to place them in a sealed container (e.g., a "perky box") with a desiccant, such as silica gel. Due to its brittleness and softness, it should be avoided to store it in contact with harder minerals.