Nitrocalcite

Chemical formula: Ca(N⁵⁺O₃)₂(H₂O)₄

Hydrated calcium nitrate, forming efflorescences and coatings in caves and on calcareous rocks in arid climates.

## Characteristics Nitrocalcite is a rare mineral from the nitrate group, being a hydrated calcium nitrate. It most commonly occurs as silky, fibrous aggregates, crusts, and efflorescences on cave walls, in fissures of calcareous rocks, and in soils in dry, desert regions. It rarely forms needle-like or hair-like crystals visible to the naked eye. It is a very delicate and hygroscopic mineral, meaning it readily absorbs moisture from the air. ## Physical Properties Nitrocalcite is very soft, with a Mohs hardness of approximately 2. It exhibits a vitreous to silky luster in the case of fibrous aggregates. It is transparent to translucent. Due to its hygroscopicity, it can become liquid in a humid environment. ## Colors and Varieties It is usually colorless or white. Impurities can give it grayish or yellowish hues. No distinct color varieties or commercial varieties are recognized. ## History and Name The name nitrocalcite refers to its chemical composition - "nitro" from nitrogen (nitrum) and "calcite" from calcium (calx), reflecting its nature as calcium nitrate. It is a mineral known for a long time, historically described as "calcium saltpeter" or "wall saltpeter" due to its appearance as efflorescences on walls and in caves. ## Uses Due to its rarity, delicacy, and water solubility, nitrocalcite has no practical industrial application. It is solely an object of collector and scientific interest.

Properties

Mohs hardness: 1-2
Color: White, grey; colourless in transmitted light
Luster: Vitreous
Streak: White
Density: 1.90
Cleavage: observed
Fracture: Uneven
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Nitrocalcite is most easily recognized by its characteristic occurrence as silky, fibrous efflorescences in dry caves and on calcareous rocks. Key diagnostic features include extreme softness, water solubility (the specimen "melts" on contact with moisture), and a salty, bitter taste. It is also very light. ## Distinguishing from Similar Minerals It can be confused with other soluble nitrates or sulfates forming efflorescences, such as nitratine, niter (potassium nitrate), or gypsum. It is distinguished from gypsum by its immediate solubility in water. From niter and nitratine, it can be distinguished by chemical tests for the presence of calcium, although this is difficult in field conditions. However, the characteristic silky, needle-like aggregates are quite typical for nitrocalcite. ## Crystal Forms Very rarely, it forms visible, elongated, needle-like crystals with a rhombic cross-section, belonging to the monoclinic system. It usually occurs in the form of fibrous aggregates, radial aggregates, crusts, and loose efflorescences.

Geological environment

## Genesis Nitrocalcite is a secondary mineral. It forms as a result of the reaction of nitrate-rich solutions with calcium-rich rocks, such as limestones and marbles, or with concrete and mortar. Nitrates originate from the decomposition of organic matter (e.g., bat guano in caves) or from bacterial processes in the soil. It crystallizes through water evaporation in dry, sheltered places. ## Mineral Associations It often co-occurs with other cave minerals and nitrates, such as niter (potassium nitrate), nitratine, gypsum, epsomite, as well as calcite and aragonite. ## Localities Significant localities are found in dry caves worldwide. It occurs in many caves in Kentucky (USA), including the famous Mammoth Cave. It is also found in caves in Italy (e.g., near Vesuvius), Spain, Russia (Urals), as well as in arid regions of Chile (Atacama Desert) and South Africa.

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, visible crystals in the form of needle-like or hair-like aggregates, which is a great rarity. Rich, "fluffy" efflorescences covering a significant area of the host rock are also attractive. Purity (snow-white color) and lack of damage to the delicate fibers are crucial. The specimen must be stable and properly protected from moisture. ## Popular Localities Classic and prized specimens come from caves in Kentucky, USA. Material from Spanish caves and the Vesuvius region in Italy is also known among collectors.

Care and storage

## Cleaning The mineral is extremely soluble in water and very delicate. It must absolutely not be cleaned with water or any liquids. The only permissible method is very careful dust removal using a soft brush or a stream of dry air at low pressure. ## What to Avoid Any contact with water, alcohol, and other liquids, which will immediately dissolve it, must be avoided. It is very hygroscopic, so it must be protected from atmospheric moisture. Temperature changes and storage in humid rooms should be avoided. It is very soft and brittle, susceptible to the slightest mechanical damage. ## Storage Nitrocalcite specimens require storage in hermetically sealed containers (e.g., "perky box" type) with the addition of a moisture-absorbing agent (e.g., silica gel). Display should only take place in sealed display cases with controlled, low humidity.