Keystoneite

Chemical formula: Mg₀.₅Ni²⁺Fe³⁺(Te⁴⁺O₃)₃·₄H₂O

Keystoneite is a very rare magnesium, nickel, and iron tellurite, forming tiny, acicular crystals of a characteristic golden-yellow color.

## Characteristics Keystoneite is a hydrated magnesium, nickel, and iron tellurite, belonging to the zemannite group. It occurs as very small, hexagonal, prismatic or acicular crystals, rarely exceeding 1 mm in length. These crystals often form radial aggregates or rosettes. Its most characteristic feature is its intense, golden-yellow color. ## Physical Properties This mineral is translucent and exhibits a vitreous luster. Due to the microscopic size of the crystals, precise determination of hardness and density is impossible; the density calculated from the formula and cell parameters is approximately 3.85 g/cm³, however, the measured value is not known. ## Colors and Varieties Keystoneite occurs in a uniform, golden-yellow color. No color varieties or commercial varieties are known. ## History and Name The mineral's name comes from its discovery locality – the Keystone Mine in Boulder County, Colorado, USA. It was first described in 1988 by J.A. Back, P.J. Dunn, E.E. Foord, J.E. Taggart Jr., and G.K. Hite. This is its type locality. ## Uses Keystoneite has no industrial application. Its significance is purely scientific and collectible, being a coveted object for collectors specializing in rare minerals or tellurium minerals.

Properties

Luster: Vitreous
Streak: Pale yellow-green
Density: 0
Transparency: Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Keystoneite can be identified by its characteristic golden-yellow color, hexagonal habit of small, acicular crystals, and occurrence in radial aggregates. Its origin from the type locality, where it coexists with other tellurides and tellurites, is also key. ## Distinguishing from Similar Minerals It may be confused with other minerals from the zemannite group, such as zemannite or moctezumite, which have a similar habit and color. Definitive differentiation requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). Its specific geological environment (oxidized tellurium deposits) distinguishes it from other yellow secondary minerals. ## Crystal Forms It forms elongated, prismatic or acicular crystals with a hexagonal outline. These crystals most often occur as radial or stellate aggregates, as well as single needles grown on the host rock.

Geological environment

## Genesis Keystoneite is a secondary mineral, forming in the oxidation (weathering) zone of hydrothermal ore deposits rich in tellurium. It forms in rock cavities and fractures as a result of alterations of primary tellurides. ## Mineral Associations At the type locality (Keystone Mine), it coexists with other tellurium minerals, such as native tellurium, sylvanite, petzite, hessite, altaite, as well as quartz, pyrite, marcasite, galena, chalcopyrite, zemannite, moctezumite, and other rare tellurites. ## Localities The only confirmed and described occurrence of keystoneite in the world is its type locality: Keystone Mine, Magnolia Mining District, Boulder County, Colorado, USA.

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued keystoneite specimens are those with well-formed, sharply terminated crystals of an intense, golden-yellow color. Rich, radial aggregates that stand out clearly from the host rock (most often quartz) are particularly sought after. Due to the microscopic size of the crystals, their abundance and aesthetic arrangement on the matrix are crucial. ## Popular Localities The only source of keystoneite specimens is its type locality – the Keystone Mine in Colorado, USA. This mine is a historical and currently inaccessible locality, which makes material from this location extremely rare and prized in the collector's market.

Care and storage

## Cleaning Keystoneite specimens are extremely delicate and brittle. Mechanical cleaning should be avoided. If absolutely necessary, compressed air can be used from a safe distance to remove dust. Contact with water or other liquids is not recommended. ## What to Avoid Contact with chemicals, acids, and detergents must be strictly avoided. Crystals are sensitive to shocks, ultrasound, and rapid temperature changes. Specimens should not be exposed to direct sunlight, which may cause them to fade. ## Storage Keystoneite specimens should be stored in stable conditions, preferably in sealed, padded "micromount" boxes, to protect them from dust, mechanical damage, and humidity changes. Display should be away from sources of vibration and direct light.