TSUMEB - Hörnesite

Mineral Species

Hörnesite

Type Locality

Composition

Mg₃(AsO₄)₂·8H₂O

Crystal System

Monoclinic

Status at Tsumeb

Believed valid

Abundance

Extremely rare

Distribution

Second oxidation zone

Paragenesis

Supergene

Entry Number

Species; TSNB170

General Notes

Hörnesite was first reported from Tsumeb by Schmetzer et al. (1983), who noted its occurrence as mixed crystals with köttigite. It also occurs as sprays (to 0.5 mm) of minute, colourless acicular crystals.

Keller (1984) emphasised that hörnesite and köttigite are visually indistinguishable and that analysis is required for identification. He considered both minerals to be part of a rare arsenate paragenesis forming under acid to neutral pH conditions and relatively low Eh. Such conditions were ideally developed in cavities in sulphide ore in the second oxidation zone.

Associated Minerals

adamite; austinite; cerussite; claudetite (?); conichalcite; dolomite; gaitite; helmutwinklerite; johillerite; keyite; koritnigite; köttigite; ludlockite; o’danielite; prosperite; quartz; smithsonite; stranskiite; tennantite-(Zn); warikahnite; willemite; zincolivenite