Selenite is a translucent, crystalline form of the common mineral gypsum, valued for its striking appearance and clarity. Its physical characteristics, especially its interaction with moisture, often cause confusion. Because selenite is soft and has a glassy luster, people frequently wonder if it can be safely exposed to water without damage. Understanding its chemical structure and solubility is necessary for proper care. This article explains selenite’s relationship with water.
Understanding the Mineral Selenite
Selenite is chemically classified as calcium sulfate dihydrate. This means its structure includes two molecules of water locked within its mineral lattice, a composition shared by all varieties of gypsum. Selenite is an evaporite mineral, typically forming as water bodies evaporate and leave behind dissolved salts.
The mineral is known for its extreme softness, registering only a 2 on the Mohs hardness scale. It can be scratched easily even with a fingernail. This low hardness makes it vulnerable to physical and chemical changes. Structurally, selenite forms in monoclinic crystals that often exhibit a tabular or prismatic habit.
The Specifics of Selenite Solubility in Water
The direct answer is that selenite is water-soluble, though only slightly. Dissolution occurs because water molecules slowly pull apart the calcium and sulfate ions that form the crystal structure. This chemical process is governed by the mineral’s low solubility product.
This low solubility means only a limited amount of the mineral can dissolve in a given volume of water before the solution becomes saturated. For example, at room temperature, only about 0.9 to 2.5 grams of selenite will dissolve in one liter of pure water. While this technically makes it soluble, the rate of dissolution is extremely slow under typical conditions.
Prolonged submersion in water, even for a few hours, will gradually cause the surface of the selenite to erode, resulting in flaking and a loss of its characteristic luster. Unlike most solids, the solubility of calcium sulfate slightly decreases as water temperature increases past room temperature. The risk of damage is cumulative, meaning repeated or long-term exposure to moisture will inevitably lead to surface degradation.
Handling and Care Considerations
Translating the science of slight solubility into practical care means avoiding all forms of submersion and prolonged exposure to moisture. Never soak selenite for cleaning or for any other purpose, as this will lead to pitting, erosion, and a noticeable reduction in the crystal’s size and sheen over time. The low Mohs hardness means its surface is already delicate, and exposure to water exacerbates its fragility.
For routine cleaning, the safest method is to use a soft, dry cloth or a gentle brush to dust the surface. If a spot clean is absolutely necessary, a minimally damp cloth can be used for a quick wipe. The crystal should be dried immediately afterward to prevent the water from initiating the slow dissolution process on the surface.
Environmental factors like high humidity and storage near water sources also pose a long-term threat. Storing selenite in a consistently moist environment, such as a bathroom, will contribute to its slow deterioration. To ensure the crystal’s longevity and maintain its clarity, always keep it in a dry location away from direct contact with liquids.