Monohydrocalcite

Chemical formula: CaCO<sub>3</sub>·H<sub>2</sub>O

Hydrated calcium carbonate, most often forming spherical or fibrous aggregates in low-temperature environments, often associated with biological activity.

## Characteristics Monohydrocalcite is a hydrated calcium carbonate, chemically a monohydrate of calcite. It rarely forms distinct, single crystals, most often occurring as spherulitic (spherical) or fibrous aggregates. It can also be found as fine-grained, soft, and porous masses resembling chalk or milky speleothems. Its structure is unstable and can recrystallize over time into more stable calcite or aragonite, losing water in the process. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2 to 3. It has a vitreous or dull luster, depending on its form. It is translucent to opaque. Its density is lower than that of anhydrous calcite, at approximately 2.4-2.5 g/cm³. ## Colors and Varieties Monohydrocalcite is typically white, colorless, grayish, or yellowish. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral's name, given in 1964 by George M. Stanley and Richard C. Tenant, directly refers to its chemical composition – it is a monohydrate of calcite, meaning calcite with one water molecule. It was first described based on material from Lake Issyk-Kul in Kyrgyzstan and Lake Lahontan in Nevada, USA. ## Uses Monohydrocalcite has no significant industrial application. It is primarily an object of scientific interest (as an indicator of specific sedimentary and biomineralization conditions) and a collector's item, although its specimens are rarely found on the market.

Properties

Mohs hardness
2-3
Luster
Vitreous, Silky, Dull
Streak
White
Density
2.4-2.5
Cleavage
Perfect on {1011}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Trigonal

Diagnostic features

## Identification Monohydrocalcite is most easily identified by its characteristic forms – spherical or fibrous aggregates. It is very soft and light. A key diagnostic feature is its vigorous reaction with hydrochloric acid (it effervesces), confirming the presence of calcium carbonate. Under laboratory conditions, the presence of water in its structure can be confirmed. ## Distinguishing from Similar Minerals It can be confused with calcite, aragonite, or chalk. It differs from calcite and aragonite by significantly lower hardness and density. Chalk is usually more porous and less consolidated. Unlike gypsum, monohydrocalcite reacts vigorously with acids. ## Crystal Forms Well-formed, single crystals are extremely rare and take the form of trigonal scalenohedra or rhombohedra. It typically forms spherulites (aggregates with a radial or concentric arrangement of fibers), fibrous aggregates, as well as soft, earthy, or chalk-like masses.

Geological environment

## Genesis Monohydrocalcite is a low-temperature mineral, formed by precipitation from waters rich in calcium and magnesium ions. Its formation is often associated with biochemical processes, e.g., the activity of microorganisms (bacteria, cyanobacteria) in alkaline lake, lagoon, or soil environments. It also forms in caves as a component of some speleothems (so-called moonmilk) and in the bottom sediments of saline lakes. ## Mineral Associations It often co-occurs with calcite, aragonite, hydromagnesite, gypsum, and nesquehonite. ## Localities Significant occurrences of monohydrocalcite have been reported in the sediments of many saline and alkaline lakes worldwide, including Lake Issyk-Kul (Kyrgyzstan), Great Salt Lake and Mono Lake (USA), Lake Van (Turkey), and volcanic lakes in Australia. It also occurs in caves in Belgium, France, and South Africa. In Poland, its presence has been confirmed in the sediments of Lake Szmaragdowe in Szczecin.

Rarity

Rare

For collectors

## Quality Criteria The most prized by collectors are specimens with well-formed, spherical spherulites with a silky luster. Groups of crystals are also attractive, although these are extremely rare. The size of the aggregates and their aesthetic arrangement on the rock matrix significantly increase the value of the specimen. Due to the mineral's instability, well-preserved specimens without signs of recrystallization are particularly sought after. ## Popular Localities Classic and most famous localities from which collector's specimens originate include the area around Lake Issyk-Kul in Kyrgyzstan and Lake Lahontan in Nevada, USA. Specimens from caves, such as the Belgian grotto "Grotte de la Clamouse", are also valued.

Care and storage

## Cleaning Monohydrocalcite specimens are very delicate and sensitive to water. They should be cleaned only dry, using a soft brush or compressed air from a safe distance. Any contact with water, especially warm water, can lead to damage or transformation of the mineral. ## What to Avoid Contact with acids (even weak ones, like vinegar), which cause rapid dissolution, must be absolutely avoided. The mineral is sensitive to elevated temperatures, which accelerate its dehydration (water loss) and transformation into calcite. It should be protected from moisture and direct sunlight. ## Storage It is recommended to store specimens in dry, stable conditions, preferably in sealed containers or display cases away from sources of heat and moisture. Due to its softness and fragility, contact with harder minerals should be avoided.

External references

Sources

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