What is Roselite

Roselite is a rare cobalt arsenate mineral with the chemical formula Ca₂(Co,Mg)[AsO₄]₂·H₂O, comprising calcium, cobalt, magnesium, arsenic, oxygen, and water. It belongs to the fairfieldite group and is well known for its distinctive rose-red to pink coloration, attributed to its cobalt content. Roselite crystallizes in the monoclinic system and is sometimes referred to as α-roselite to distinguish it from β-roselite, its rarer triclinic dimorph.

This mineral typically forms in the oxidation zones of cobalt-rich hydrothermal deposits, where weathering of primary cobalt arsenides—such as skutterudite—releases cobalt and arsenate ions. These then recombine under acidic, oxidizing conditions to form secondary minerals like Roselite. It may occur as elongated prismatic crystals or spherolitic aggregates, exhibiting a vitreous to resinous luster and vibrant pink to purplish-red hues.

Roselite is named after Gustav Rose, a 19th-century mineralogist from the University of Berlin. Its aesthetic appeal and rarity make it a highly sought-after species among mineral collectors, especially those focused on cobalt and arsenate mineral assemblages.

Crystals of Roselite. From: Aghbar Mine (Arhbar Mine), Aghbar, Bou Azer District, Tazenakht, Ouarzazate Province, Souss-Massa-Draâ Region, Morocco.
Photo: Domenico Preite

Composition

Formula: Ca₂(Co,Mg)(AsO₄)₂·2H₂O

Primary Elements: Calcium (Ca), Cobalt (Co), Arsenic (As), Oxygen (O), and water molecules.

Common Substitutions: Magnesium (Mg) and sometimes Nickel (Ni) can substitute for Cobalt, creating solid-solution series with minerals such as Erythrite (Co end-member) and Fairfieldite (Mn-rich variety).

The vivid color is due primarily to cobalt content.

Pink roselite crystals on brown goethite and white orthoclase from Bou Azer, Morocco.

Properties of Roselite

Color: Rose-red to pink (cobalt-dependent), occasionally violet or orange-tinged.
Streak: Pale pink to white.
Luster: Vitreous to sub-adamantine or resinous.
Transparency: Transparent to translucent.
Crystal Habit: Prismatic or tabular monoclinic crystals; often forms spheroidal aggregates.
Hardness: 3.5–4 on Mohs scale (relatively soft, similar to fluorite).
Cleavage: Perfect on {010}, poor on {100}.
Fracture: Uneven to subconchoidal.
Density: 3.5–3.7 g/cm³ (varies with Mg/Co ratio).
Fluorescence: Some specimens fluoresce green under UV light.

Occurrence of Roselite

Roselite is a secondary mineral that forms in the oxidized zones of cobalt-arsenic-bearing hydrothermal deposits. It crystallizes from the alteration of primary cobalt arsenides, such as skutterudite (CoAs₃) and cobaltite (CoAsS), under oxidizing conditions.

Notable Localities

Bou Azzer, Morocco – The world’s premier source, producing well-crystallized roselite in association with erythrite and cobaltite.
Germany – Classic occurrences in Schneeberg and Saxony.
Namibia – Found in the Tsumeb Mine, a famous polymetallic deposit.
USA – Blackbird Mine (Idaho), and other small oxidized cobalt prospects.