Johillerite

Chemical formula: NaCu₂+MgMg₂(As₅+O₄)₃

A sodium, copper, and magnesium arsenate, forming characteristic, intensely blue or violet tabular crystals.

## Characteristics Johillerite is a rare mineral from the arsenate group, chemically classified as a sodium, copper, and magnesium arsenate. It occurs as small, but well-formed tabular crystals, less commonly short-prismatic. It typically forms druses and small crystalline aggregates. Its most distinctive feature is its intense, deep blue to violet-blue color. ## Physical Properties Johillerite crystals are transparent to translucent and exhibit a vitreous luster. The mineral's hardness on the Mohs scale is approximately 3, making it relatively soft and susceptible to scratching. Its density is significantly higher than average, at 4.29 g/cm³. The streak, or color of the powdered mineral, is light blue. ## Colors and Varieties This mineral occurs in uniform, attractive colors ranging from deep blue to violet-blue shades. No distinct color varieties or commercial forms are recognized. ## History and Name Johillerite was first described in 1980 by Paul Keller, Heinz Hess, and Pete J. Dunn. The mineral's name honors Johannes-Erich Hiller (1911–1972), a professor of mineralogy at the University of Stuttgart in Germany. The type locality, from which the first studied specimens originated, is the famous Tsumeb mine in Namibia. ## Applications Due to its extreme rarity and small crystal size, johillerite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for advanced systematic mineral collections and Tsumeb specimens.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Pale blue
Density: 4.29
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Key diagnostic features of johillerite include its intense blue or violet color, tabular crystal habit, vitreous luster, and light blue streak. However, the most important indicator is its origin – almost all specimens come from the Tsumeb mine – and its co-occurrence with other rare arsenates. ## Distinguishing from Similar Minerals Johillerite is sometimes confused with other blue secondary minerals from Tsumeb, such as keyite or linarite. Keyite typically has a more greenish-blue hue and a different crystal habit. Linarite is characterized by a brighter, more "electric" blue color and a different crystal form. Final and certain differentiation often requires advanced studies, e.g., chemical analysis. ## Crystal Forms Johillerite forms thin to thick tabular crystals, less commonly prismatic. They often occur as druses, crusts, or radial aggregates on the host rock.

Geological environment

## Genesis Johillerite is a secondary mineral that forms in the oxidation zone of complex, polymetallic hydrothermal deposits. In Tsumeb, it formed within dolomitic rocks as a result of the weathering of primary arsenic- and copper-bearing ores. ## Mineral Associations At the type locality (Tsumeb), johillerite co-occurs with other rare arsenates, such as o'danielite, keyite, ludlockite, and schultenite, as well as tennantite and chalcocite. ## Localities The most important and practically sole source of well-formed johillerite specimens worldwide is the Tsumeb mine in the Otjikoto Region, Namibia. This is the classic locality for this mineral.

Rarity

Very rare

For collectors

## Quality Criteria The collector's value of johillerite depends on several factors. Specimens with sharp, well-formed crystals of intense, deep blue color are most highly prized. The size of the crystals and their abundance on the rock matrix are also important. Aesthetic compositions with other, color-contrasting minerals from Tsumeb are particularly desirable. ## Popular Localities The only locality from which johillerite specimens of collector significance originate is the Tsumeb mine in Namibia. Specimens from this location are considered the global standard for this species.

Care and storage

## Cleaning Johillerite specimens should only be cleaned mechanically, using a soft brush or compressed air to remove dust. Contact with water should be avoided; if absolutely necessary, distilled water can be used, and the specimen should then be immediately and thoroughly dried. ## What to Avoid The mineral is sensitive to acids, in which it readily dissolves. As an arsenate, it is potentially toxic – hands should be washed after handling a specimen. Its low hardness (3 on the Mohs scale) makes it very susceptible to scratching. It should be protected from moisture, temperature fluctuations, and prolonged exposure to sunlight. ## Storage It is recommended to store specimens in closed, stable conditions, for example, in "micromount" boxes or sealed display cases. It should be isolated from harder minerals to avoid accidental mechanical damage.

Sources

IMA Mineral List (CNMNC)