Rhomboclase

Chemical formula: (H₅O₂)Fe³⁺(S⁶⁺O₄)₂·₂H₂O

Rhombochlase is a hydrated iron(III) sulfate, forming brittle, fibrous aggregates or tabular crystals, often with a pale blue or yellowish coloration.

## Characteristics Rhombochlase is a mineral from the sulfate group, chemically a hydrated iron(III) sulfate. It most commonly occurs as efflorescences, crusts, and coatings, as well as in fibrous, radial, or botryoidal aggregates. Less frequently, it forms small, tabular or bladed crystals with a rhombic outline. It is a brittle mineral and sensitive to environmental conditions, especially moisture. ## Physical Properties This mineral is characterized by low hardness, 2 on the Mohs scale, which means it is very soft. Its density is approximately 2.23 g/cm³. Crystals exhibit perfect cleavage in one direction and good cleavage in two others. The fracture is conchoidal or fibrous. The luster is vitreous, and on cleavage surfaces, it can be pearly. ## Colors and Varieties Rhombochlase can be various colors – colorless, white, gray, yellow, pale blue, or pale green. In transmitted light, it is colorless. The coloration depends on impurities and the degree of hydration. No named varieties are distinguished. ## History and Name The name rhombochlase comes from the Greek words *rhombos* (rhomb) and *klasis* (to break), referring to the rhombic shape of the crystal cross-section and its perfect cleavage. The mineral was first described in 1888 based on specimens found in Smolník, Slovakia, which is its type locality. ## Uses Rhombochlase has no industrial significance. It is solely an object of interest for collectors specializing in rare secondary minerals or mineral systematics.

Properties

Mohs hardness: 2
Luster: Vitreous
Streak: White
Density: 2.23
Cleavage: On {001} perfect; on {110} good.
Fracture: Conchoidal,Fibrous
Transparency: Transparent
Crystal system: Orthorhombic

Diagnostic features

## Identification Rhombochlase can be identified by its occurrence as fibrous or powdery efflorescences in the oxidation zones of sulfide deposits. Its very low hardness, lightness, and sensitivity to moisture are characteristic. It often has a sour, metallic taste, but taste testing is highly discouraged due to toxicity. ## Distinguishing from Similar Minerals It can be confused with other secondary sulfates such as melanterite, halotrichite, or epsomite. Melanterite usually has a more intense, green color and is also unstable. Halotrichite forms characteristic, hair-like aggregates ("stone wool"). Distinguishing it from many similar sulfates often requires chemical analysis (XRD). ## Crystal Forms Crystals, if present, are thin-tabular or bladed, with a rhombic outline. Most often, however, it forms fibrous, radial, botryoidal aggregates, as well as crusts, coatings, and powdery efflorescences.

Geological environment

## Genesis Rhombochlase is a typical secondary mineral. It forms as a result of weathering (oxidation) of sulfide minerals, mainly pyrite and marcasite, in dry and hot climates. It forms as an evaporation product of acidic mine waters. It is an unstable mineral, forming under specific conditions, often in old mine workings. ## Mineral Associations It co-occurs with other iron and metal sulfates, such as melanterite, copiapite, voltaite, halotrichite, szomolnokite, as well as with gypsum, native sulfur, and limonite. ## Localities Key localities worldwide include Smolník (Slovakia - type locality), Rammelsberg mine in Goslar (Germany), Iron Monarch (South Australia), Jerome in Arizona (USA), and numerous sites in Chile (e.g., Chuquicamata) and Italy (Vulcano island).

Rarity

Rare

For collectors

## Quality Criteria Most valued by collectors are well-formed, even if small, transparent crystals with a distinct rhombic outline. Rich, fibrous aggregates with an attractive, pale blue or yellow color, preserved on the original matrix, are also highly prized. Due to the mineral's instability, the condition of the specimen is crucial – the absence of signs of dehydration (disintegration) is a necessary condition. ## Popular Localities Classic and most sought-after specimens come from the type locality in Smolník, Slovakia. High-quality material, especially in the form of fibrous aggregates, is also supplied from the Rammelsberg mine in Germany and from some localities in Arizona (USA).

Care and storage

## Cleaning Rhombochlase specimens are extremely sensitive to water and moisture. They must absolutely not be cleaned wet. The only permissible method is to very gently remove dust with a soft brush or a stream of compressed air from a distance. ## What to Avoid Avoid contact with water, water vapor, and any moisture, as the mineral readily dissolves and undergoes chemical transformations. It is unstable in dry air, where it can dehydrate (lose water) and crumble into powder. It should be protected from temperature changes and direct sunlight. ## Storage Rhombochlase specimens require storage in hermetically sealed containers (e.g., "membrane boxes" or airtight acrylic boxes), preferably with a desiccant (silica gel). Display should be in stable conditions, away from heat and moisture sources.