Selenite

Chemical formula: CaSO₄·₂H₂O

Selenite is a transparent, crystalline variety of gypsum, valued for its pearly luster and often occurring as elongated crystals.

## Characteristics Selenite is a crystalline variety of gypsum, characterized by its transparency or translucency. It often forms elongated, tabular, or prismatic crystals that can reach significant sizes. The surface of the crystals may be striated parallel to their elongation. It also occurs in fibrous forms, creating so-called "desert roses" or "gypsum flowers," which are aggregates of crystals with a characteristic shape. ## Physical Properties Selenite is characterized by low hardness, making it easily scratchable. It has perfect cleavage, meaning it readily splits into thin sheets. Selenite's luster is vitreous to pearly, especially on cleavage surfaces. It is transparent to translucent, and its density is relatively low. ## Colors and Varieties Selenite is most often colorless or white, but it can take on delicate shades of gray, yellow, brown, or green, depending on impurities. Some varieties of selenite exhibit a cat's eye effect, especially in fibrous forms. There are no specific commercial names for colored varieties of selenite, apart from the general name "desert rose" for gypsum aggregates. ## History and Name The name "selenite" comes from the Greek word "selene," meaning moon, which refers to its pearly luster and delicate, moon-like glow. This mineral has been known and used since antiquity, mainly due to its transparency, which allowed it to be used as a substitute for glass in windows. ## Applications Due to its low hardness, selenite has no industrial applications requiring high strength. However, it is valued in jewelry as an ornamental stone, especially in the form of cabochons, and in sculpture for creating decorative objects. Its transparency makes it sometimes used for producing watch faces or optical components. It is also a popular collector's mineral.

Properties

Mohs hardness: 2
Luster: Szklisty, perłowy
Streak: Biała
Density: 2.32
Cleavage: Doskonała wg {010}
Fracture: Muszlowy, nierówny
Transparency: Przezroczysty do przeświecającego
Crystal system: Jednoskośny

Diagnostic features

## Identification Selenite can be identified by its transparency, pearly luster, and low hardness, which allows it to be scratched with a fingernail. It often occurs as elongated, tabular crystals that are flexible but not elastic. Perfect cleavage, allowing it to be easily split into thin sheets, is also characteristic. ## Distinguishing from similar minerals Selenite is sometimes confused with calcite or quartz, but its low hardness (easily scratched with a fingernail) distinguishes it from these harder minerals. Calcite reacts with hydrochloric acid, which selenite does not. Quartz is much harder and does not have such perfect cleavage. From other varieties of gypsum, selenite is distinguished by its transparency and well-formed crystals. ## Crystal forms Selenite most often forms elongated, tabular, or prismatic crystals, often with characteristic striations parallel to the elongation axis. It also occurs as fibrous aggregates, forming so-called "desert roses" or "gypsum flowers," which are radially arranged crystals. These aggregates can take on rosette or spherical forms.

Geological environment

## Genesis Selenite forms mainly in evaporitic environments, i.e., as a result of the evaporation of saline waters, such as lakes, lagoons, or seas. It forms in sedimentary evaporitic rocks, often in association with halite and anhydrite. It can also crystallize in the oxidation zones of sulfide deposits and in caves, where it forms speleothems and crystals. ## Mineral associations Selenite often co-occurs with other evaporitic minerals, such as halite, anhydrite, calcite, aragonite, as well as various clay minerals and sand. In caves, it can be found with calcite and aragonite. ## Localities Significant deposits of selenite occur in many places around the world. The most famous include the Naica caves in Mexico, where gigantic selenite crystals have been found. Other important localities include Spain (e.g., Pilar de Jaravía mine), the United States (e.g., Oklahoma, Utah), Australia, Greece, Italy, and Poland.

Rarity

Common

For collectors

## Quality criteria The most prized selenite specimens are those with high transparency, intense luster, well-formed, and undamaged crystals. Large, single crystals and aesthetic aggregates, such as "desert roses," are particularly sought after. The purity of the mineral, lack of inclusions, and mechanical damage are also important. Specimens with rare colors or a cat's eye effect also increase their collector's value. ## Popular localities The most prized selenite specimens come from the Naica caves in Mexico, famous for their gigantic crystals. Other popular sources include Spain, where beautiful, transparent crystals are found, and the United States, known for its "desert roses."

Care and storage

## Cleaning Selenite is a soft and delicate mineral, so it should be handled with care. For cleaning, it is recommended to use a soft, dry cloth or brush to remove dust. For heavier soiling, it can be gently wiped with a damp cloth and then immediately dried. ## What to avoid Avoid prolonged contact of selenite with water, as this can cause it to dissolve or damage its surface. Do not use any chemical cleaning agents, acids, or detergents, which can irreversibly damage the mineral. Selenite is also susceptible to scratches, so avoid contact with harder objects. High temperatures and sudden temperature changes can also negatively affect its structure. ## Storage Selenite is best stored in a dry place, away from direct sunlight, which can cause some varieties to fade. Due to its softness, it is recommended to store it separately from other, harder minerals to prevent scratches. It is ideally suited for display in enclosed showcases, which protect it from dust and mechanical damage.