Orschallite
Chemical formula: Ca<sub>3</sub>(S<sup>4+</sup>O<sub>3</sub>)<sub>2</sub>S<sup>6+</sup>O<sub>4</sub>·12H<sub>2</sub>O
Orschallite is an extremely rare, hydrated calcium sulfite-sulfate, forming colorless, acicular crystals on mine dumps.
Description
## Characteristics Orschallite is a mineral from the sulfite group, chemically classified as a hydrated calcium sulfite-sulfate. It occurs as very small, colorless, and transparent crystals with a prismatic or acicular habit. These crystals rarely occur individually, most often forming radial or fascicular aggregates and crusts, resembling tiny brushes or tufts. Due to the small size of the crystals, it is a typical micromount mineral, whose beauty can only be appreciated under magnification. ## Physical Properties This mineral is very soft and delicate. It is characterized by a vitreous luster and is completely transparent. Its density is low, approximately 2.03 g/cm³ (calculated value). It is unstable under atmospheric conditions, especially in humid air, where the sulfite it contains can undergo oxidation. ## Colors and Varieties Orschallite is colorless. No colored varieties or trade names are known. ## History and Name The mineral's name comes from its discovery locality – the Orschall quarry in the St. Andreasberg area in the Harz Mountains (Lower Saxony, Germany). It was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 2017. ## Uses Orschallite has no industrial application. It is solely an object of scientific research and a valuable acquisition for specialized collectors of rare minerals.
Diagnostic features
## Identification Identification of orschallite is based on its characteristic form – acicular or prismatic crystals forming radial aggregates. The locality (mine dumps in St. Andreasberg) and co-occurring minerals are crucial. It is very soft and reacts violently with hydrochloric acid, releasing sulfur dioxide with a characteristic odor. Due to its rarity and small size, definitive confirmation requires advanced analytical methods. ## Distinguishing from Similar Minerals Orschallite can be visually confused with other white, acicular secondary minerals, such as gypsum, thaumasite, or aragonite. It differs from gypsum by its violent reaction with acid. Thaumasite also forms acicular crystals but has a different chemical composition (contains silicon). Aragonite is much harder. Certain differentiation most often requires chemical analysis (EDS) or X-ray analysis (XRD). ## Crystal Forms Crystals are elongated, with a prismatic to acicular habit. They usually occur in the form of radial or chaotically arranged aggregates, as well as crusts and tufts on the surface of the host rock.
Geological environment
## Genesis Orschallite is a secondary mineral, formed under very specific low-temperature conditions. It forms on old mine dumps as a result of the reaction of sulfur-rich solutions (from the weathering of sulfides) with calcium carbonate (calcite) in a low-oxygen environment, which allows for the stabilization of the sulfite ion (S⁴⁺). ## Mineral Associations This mineral occurs in association with other secondary minerals such as calcite, gypsum, and thaumasite. ## Localities The only confirmed locality of orschallite in the world is its type locality – the Orschall quarry in St. Andreasberg, in the Harz Mountains, Germany.
Rarity
Extremely rare
Collector aspects
## Quality Criteria The quality of orschallite specimens, being a micromount mineral, is assessed based on several criteria. Specimens with well-formed, sharp, and undamaged crystals are most highly valued. The abundance of the mineral on the rock matrix and the aesthetics of the aggregates are also important – distinct, radial "stars" or dense "brushes" are particularly sought after. Contrast with the color of the host rock also enhances visual appeal. ## Popular Localities The only source of orschallite specimens is its type locality in Germany. Material from this location is extremely difficult to obtain and almost exclusively goes to institutional collections and leading private collectors specializing in mineral systematics.
Care and storage
## Cleaning Orschallite specimens require extreme caution. Cleaning should be limited to dry methods. Only very gentle dust removal with a soft brush (e.g., sable) or blowing it off with compressed air from a safe distance is permissible. ## What to Avoid Contact with water and all liquids that can dissolve the mineral or initiate chemical reactions must be absolutely avoided. The mineral is unstable in humid air, so it must be protected from moisture, which causes its slow decomposition (oxidation of sulfite to sulfate). High temperatures (risk of dehydration), direct sunlight, and any shocks and impacts should also be avoided due to its fragility. ## Storage Storing orschallite requires special conditions. The safest way is to place the specimen in a sealed, closable box (a "perky box" type) along with a moisture-absorbing agent (desiccant, e.g., silica gel). Specimens should be stored at a stable temperature, away from light.