Metathénardite

Chemical formula: Na₂S⁶⁺O₄

Metathenardite is an anhydrous sodium sulfate, a mineral formed by the dehydration of thenardite, often found in arid and hot climates.

## Characteristics Metathenardite is an anhydrous sodium sulfate, most commonly formed by the dehydration of thenardite. This process occurs due to changes in temperature and humidity. It usually forms granular, powdery, or earthy aggregates, as well as crusts and efflorescences. It is rarely observed in the form of well-developed crystals, which, if present, are inherited from the primary thenardite form (pseudomorphs). ## Physical Properties This mineral is characterized by a hardness ranging from 3-4 on the Mohs scale. It has a vitreous luster and is typically transparent to translucent. Its density is approximately 2.67 g/cm³. It is brittle, and its fracture is uneven to conchoidal. ## Colors and Varieties Metathenardite is most often colorless or white. Impurities can give it gray, yellowish, or brownish hues. No named varieties are distinguished. ## History and Name The name "metathenardite" was introduced by A. Lacroix in 1905. It refers to its relationship with thenardite (Greek *meta* - after, beside) and to the French chemist Louis Jacques Thénard, after whom thenardite was named. The mineral was known earlier, but Lacroix formally described its relationship with thenardite and the transformation process. ## Uses Metathenardite, like thenardite, is a source of sodium sulfate, used in the chemical industry, among others, for the production of detergents, paper (sulfate process), and glass. However, it primarily has scientific and collector's significance.

Properties

Mohs hardness
3
Color
Colorless, white, light-blue, greenish, yellowish, grayish or brownish,
Luster
Vitreous
Streak
White
Density
2.72
Cleavage
Good on {111}
Fracture
Irregular/Uneven
Transparency
Transparent
Crystal system
Hexagonal

Diagnostic features

## Identification A key diagnostic feature of metathenardite is its water solubility. In field conditions, it can be recognized by its occurrence as white, powdery efflorescences in dry, desert environments. It often forms pseudomorphs after thenardite crystals. ## Distinguishing from Similar Minerals Metathenardite can be confused with thenardite, mirabilite, gypsum, or halite, with which it often co-occurs. It differs from thenardite in its crystallographic system (orthorhombic vs. monoclinic thenardite at low temperatures). It differs from mirabilite by the absence of "water" in its structure and greater stability in dry air. It is much softer than gypsum. It differs from halite by its bitter, not salty, taste. ## Crystal Forms Metathenardite crystallizes in the orthorhombic system. It rarely forms its own well-developed crystals. Most often, it occurs as a pseudomorph after orthorhombic thenardite crystals (stable above 32.4 °C), retaining their external form, e.g., pyramidal or tabular.

Geological environment

## Genesis Metathenardite is a secondary mineral, formed by the dehydration of thenardite under conditions of low humidity and elevated temperature. It is a typical mineral for evaporitic environments, such as drying salt lakes (playas), salt flats, and deserts. It can also form as a product of volcanic exhalations (fumaroles). ## Mineral Associations It most often co-occurs with thenardite, mirabilite, blödite, epsomite, gypsum, and halite. ## Localities Significant deposits and occurrences of metathenardite are found in desert regions and evaporite deposits. Important localities include Soda Lake and Searles Lake in California (USA); Camp Verde in Arizona (USA); Great Salt Lake in Utah (USA); the Aldan River valley in Siberia (Russia); Wadi el-Natrun in Egypt; as well as in arid regions of Chile, Peru, and Italy (Vesuvius).

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are well-formed pseudomorphs of metathenardite after large, sharp thenardite crystals. Transparency, purity (lack of mud or clay inclusions), and aesthetic association with other salt minerals, such as red halite, are important. Specimens in the form of loose aggregates have little collector's value. ## Popular Localities Among the most famous localities from which high-quality collector specimens originate are the borate and salt deposits in California, especially Searles Lake in San Bernardino County. Specimens from this locality are often exemplary for this mineral.

Care and storage

## Cleaning Metathenardite is water-soluble, therefore water or any water-based liquids must not be used for cleaning. Specimens can only be dry-cleaned using a soft brush or compressed air to remove dust. ## What to Avoid Contact with water, water vapor, and high humidity must be strictly avoided, as these can cause its dissolution or transformation into other minerals. It should also be protected from sudden temperature changes. ## Storage Metathenardite specimens require storage in dry conditions. It is best to place them in a sealed, airtight container (e.g., a membrane box or plastic box) along with a desiccant (e.g., silica gel).

External references

Sources

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