Ferrilotharmeyerite

Chemical formula: CaZn<sup>2+</sup>Fe<sup>3+</sup>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)·H<sub>2</sub>O

Ferrilotharmeyerite is a rare arsenate of the tsumcorite group, forming brownish-red, tabular crystals with a vitreous luster.

## Characteristics Ferrilotharmeyerite is a hydrated calcium, zinc, and iron arsenate, belonging to the tsumcorite group. This mineral forms very small, tabular or bladed crystals, which typically do not exceed 0.2 mm in length. These crystals often occur as rosette-like or radial aggregates, as well as thin coatings and crusts on host rocks. Its appearance is distinctive, although due to the small size of the crystals, magnification is often required for full identification. ## Physical Properties Ferrilotharmeyerite crystals exhibit a vitreous luster. They are brittle, and their Mohs hardness is approximately 4.5. Due to the small size of the crystals, precise determination of physical properties such as density is difficult; it has been calculated as 4.45 g/cm³. ## Colors and Varieties This mineral occurs in shades from brownish-red to reddish-brown. No color varieties or commercial varieties are known. ## History and Name Ferrilotharmeyerite was first described in 2002 by J. Schlüter, K.-H. Klaska, G. Adiwidjaja, and G. Gebhard. Its name refers to its chemical composition – the dominance of iron (ferri) – and its relationship to lotharmeyerite, in which iron replaces zinc in one of the structural positions. It was approved as a new mineral by the International Mineralogical Association (IMA) under number IMA2001-049. The type locality, from which the first described specimens originate, is the Tsumeb mine in Namibia.

Properties

Mohs hardness
4.5
Luster
Vitreous
Streak
Light brown
Density
4.45
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of ferrilotharmeyerite in the field is practically impossible due to its rarity and microscopic crystal size. In a collection, it is recognized by its characteristic, brownish-red, tabular crystals forming rosette-like aggregates. Definitive identification requires advanced analytical methods, such as Raman spectroscopy or EDS analysis, to confirm its chemical composition. ## Differentiation from similar minerals Ferrilotharmeyerite can be confused with other minerals from the tsumcorite group, such as lotharmeyerite, tsumcorite, or gartrellite, which occur in similar environments and have a comparable appearance. Distinguishing them based on visual characteristics is impossible and requires chemical analysis to determine the dominant elements. ## Crystal forms The mineral forms very small, thin, tabular crystals with a hexagonal or rhombic outline, often elongated and flattened. These crystals combine into radial or rosette-like aggregates, and also form thin crusts.

Geological environment

## Genesis Ferrilotharmeyerite is a secondary mineral, formed in the oxidation zones (gossans) of polymetallic ore deposits, especially those rich in arsenic, zinc, and iron. It forms as a result of the weathering of primary ore minerals under conditions of low temperature and pressure. ## Mineral associations This mineral co-occurs with other rare arsenates, such as beudantite, tsumcorite, lotharmeyerite, gartrellite, as well as quartz, goethite, and tennantite. ## Localities The most important and only confirmed locality for ferrilotharmeyerite is its type locality – the famous Tsumeb mine in the Oshikoto region of Namibia. This is the only site from which well-studied and described specimens of this mineral originate.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a ferrilotharmeyerite specimen is primarily assessed based on the richness and visibility of the crystals on the rock matrix. Most desirable are specimens with well-formed, sharp, rosette-like aggregates of intense, brownish-red color, contrasting with the substrate. Due to the microscopic nature of the mineral, even small but crystal-rich clusters are highly valued by collectors specializing in rare minerals and "micromount" specimens.

Care and storage

## Cleaning Due to extreme brittleness and small crystal size, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust. Avoid contact with water and any chemicals. ## What to avoid Avoid ultrasonic cleaners, chemicals, acids, and sudden temperature changes. The mineral is very brittle, so it should be protected from impacts and vibrations. ## Storage Ferrilotharmeyerite specimens should be stored under stable conditions, in sealed "micromount" boxes, to protect them from mechanical damage and dust. Avoid exposure to direct sunlight, although its light sensitivity is not known.

Sources

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