Mirabilite

Chemical formula: Na₂S⁶⁺O₄·₁₀H₂O

Mirabilite is a hydrated sodium sulfate, forming unstable, colorless crystals that quickly lose water in dry air and transform into a white powder (thenardite).

## Characteristics Mirabilite, also known as Glauber's salt, is a hydrated sodium sulfate. It forms colorless or white, transparent to translucent crystals with a prismatic, acicular, or tabular habit. It also occurs as efflorescences, crusts, and granular or fibrous aggregates. A characteristic feature of mirabilite is its instability in low humidity conditions – crystals rapidly lose water from their crystal lattice, disintegrating into a white, powdery thenardite (anhydrous sodium sulfate). This process is reversible. ## Physical Properties Mirabilite is a very soft mineral, with a Mohs hardness of only 2-2.5. It has a vitreous luster and is relatively light, with a density of about 1.48 g/cm³. It is brittle, and its fracture is conchoidal. It exhibits perfect cleavage in one direction. It has a salty, bitter taste and is soluble in water. ## Colors and Varieties It is usually colorless or white. Impurities can impart yellowish, grayish, or greenish hues. No named varieties are distinguished, and its commercial name – Glauber's salt – refers to its synthetic or purified counterpart. ## History and Name The mineral's name comes from the Latin "sal mirabilis" (miraculous salt), given to it in the 17th century by Johann Rudolf Glauber, who discovered its purgative properties. As a mineral, it was first described in 1845 by Wilhelm Haidinger. ## Uses Historically, Glauber's salt (natural mirabilite or its synthetic equivalent) was widely used in medicine as a laxative. Currently, sodium sulfate obtained from mirabilite is used in the chemical industry, mainly for the production of detergents, paper (kraft process), glass, and in the textile industry.

Properties

Mohs hardness: 1.5-2.5
Color: Colourless, white; colourless in transmitted light
Luster: Vitreous
Streak: White
Density: 1.464
Cleavage: On {100} perfect; also reported on {001}, {010}, and {011}.
Fracture: Conchoidal
Transparency: Transparent,Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of mirabilite is its instability in dry air – when left exposed, it quickly becomes covered with a white, powdery thenardite efflorescence. Other features include low hardness (can be scratched with a fingernail), solubility in water, salty-bitter taste, and vitreous luster. It often forms characteristic efflorescences and crusts. ## Distinguishing from Similar Minerals - **Halite (rock salt)**: Has a similar appearance and is soluble in water, but has a salty taste (without a bitter aftertaste), perfect cubic cleavage, and is stable in dry air. - **Gypsum**: Is also soft, but insoluble in water and stable under atmospheric conditions. - **Thenardite**: Is the dehydration product of mirabilite, usually occurring as powdery coatings on mirabilite or pseudomorphs. It is harder (2.5-3) and has a higher density. - **Epsomite**: Also forms unstable, water-soluble efflorescences, but crystallizes in the orthorhombic system and has a different chemical composition (magnesium sulfate). ## Crystal Forms Mirabilite crystallizes in the monoclinic system, forming short, prismatic or tabular crystals, often terminated by a dipyramid. It commonly occurs as fibrous, acicular, granular aggregates, and as efflorescences, coatings, and crusts on rock or soil surfaces.

Geological environment

## Genesis Mirabilite is a typical evaporite mineral. It forms as a result of the evaporation of saline lakes, salt pans, and playa waters, especially in dry and cold climates, because its crystallization from solution is more efficient at low temperatures (below 32.4 °C). It also forms as a product of volcanic sublimation (exhalations), in caves (as efflorescences from seeping sulfate-rich waters), and as a deposit around hot springs. ## Mineral Associations It most commonly co-occurs with other evaporites, such as thenardite (often as a product of its dehydration), gypsum, halite, epsomite, trona, and borax. ## Localities Significant deposits and occurrences of mirabilite are found in many places around the world. The most important include saline lakes in the western USA (e.g., Great Salt Lake in Utah, Searles Lake in California), in Canada (Saskatchewan), Spain (near Madrid), Austria (Hallstatt), Egypt, Kazakhstan (Kara-Bogaz-Gol Bay), and Siberia. It also occurs in volcanic lake sediments and in Antarctica (e.g., in the McMurdo Dry Valleys).

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are large, well-formed, transparent, and inclusion-free mirabilite crystals. Due to the mineral's instability, specimens that have been properly preserved and have not undergone dehydration have significantly higher value. Crystal groups and specimens showing association with other evaporites are also attractive. Since most specimens are white or colorless aggregates, color intensity is not a key criterion, unless it is a rare hue caused by admixtures. ## Popular Localities Classic crystalline specimens come from historical localities in Austria (Hallstatt) and Spain (near Madrid). Large quantities of mirabilite, though less often in the form of well-developed collector crystals, are obtained from drying lakes in California (Searles Lake) and Utah (Great Salt Lake) in the USA.

Care and storage

## Cleaning Mirabilite is practically not cleaned wet, as it is soluble in water. Any contact with water, even a damp cloth, will damage or dissolve the specimen. Only very gentle dusting with a dry, soft brush is permissible to remove dust. ## What to Avoid Water and high humidity must be absolutely avoided, as they lead to the dissolution of the mineral. Equally harmful is dry air (humidity below ~80%), which causes its dehydration and disintegration into thenardite. Avoid temperature changes, direct sunlight, and heat sources. The mineral is very soft and brittle, susceptible to any mechanical damage. ## Storage Mirabilite specimens require specialized storage conditions. They are best kept in tightly sealed containers (e.g., plastic boxes with a gasket) that maintain stable, high humidity. A small water reservoir or a soaked material can be placed inside the container to prevent drying out. Store in a cool, dark place, away from vibrations and heat sources.