Robertsite

Chemical formula: Ca₂Mn³⁺₃O₂(PO₄)₃·₃H₂O

Robertsite is a rare hydrated calcium manganese phosphate, forming characteristic, radial aggregates of a reddish-brown color.

## Characteristics Robertsite is a mineral from the mitridatite group, chemically classified as a hydrated calcium manganese oxyphosphate. It occurs as small, spherical or hemispherical aggregates with a radial structure, which can reach up to 2 mm in diameter. Less commonly, it forms druses of tiny, platy crystals. Its appearance is distinctive, often with a silky or slightly brownish luster on the surface of spherulitic clusters. ## Physical Properties This mineral is relatively soft, with a hardness of 3.5 on the Mohs scale. It is translucent and has a density of 3.13 g/cm³. It exhibits distinct cleavage in one direction. Its luster is described as vitreous to silky. ## Colors and Varieties Robertsite ranges in color from red, through reddish-brown and brown with a brownish hue, to almost black. No named varieties are distinguished. ## History and Name The mineral was first described in 1974 by Paul B. Moore and Jun Ito. Its name honors Willard L. Roberts (1923–1987), curator of the Museum of Geology at the South Dakota School of Mines and Technology, in recognition of his contributions to the mineralogy of pegmatites from the Black Hills region of South Dakota. ## Uses Due to its rarity and small specimen sizes, robertsite has no industrial application. It is solely a mineral of scientific and collector interest.

Properties

Mohs hardness: 3.5
Luster: Vitreous to silky
Streak: Red-brown
Density: 3.13
Cleavage: {100}
Fracture: Micaceous
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Robertsite is most easily identified by its characteristic mode of occurrence - small, radial, spherical aggregates. Its reddish-brown color and streak of the same color are also important diagnostic features. It typically occurs in vugs in phosphate pegmatites. ## Differentiation from Similar Minerals It can be confused with other minerals from the mitridatite group, such as mitridatite or arseniosiderite, which form similar clusters. Differentiation often requires advanced analytical methods, such as X-ray diffraction (XRD). It differs from jahnsite-(CaMnMn), with which it often co-occurs, in form (jahnsite forms prismatic crystals) and color. ## Crystal Forms Robertsite crystals are very small, platy or tabular, flattened parallel to the cleavage plane. Most often, they occur as radial, spherulitic aggregates, forming spherical or hemispherical shapes on the surface of other minerals.

Geological environment

## Genesis Robertsite is a secondary phosphate mineral that forms as a result of hydrothermal alteration of primary phosphates (mainly triphylite) in phosphorus-rich zones within granitic pegmatites. It forms in the late stage of crystallization, filling vugs and fractures in the rock. ## Mineral Associations This mineral co-occurs with other secondary phosphates. At the type locality (Tip Top Mine), it was found in association with rockbridgeite, jahnsite-(CaMnMn), triphylite, zwieselite, hureaulite, and strengite. ## Localities The most important and type locality for robertsite is the Tip Top Mine in Custer County, South Dakota, USA. This is the main locality from which the best-formed specimens originate. Outside the USA, its presence has also been reported in the Bendada pegmatite in Portugal and at the Palermo No. 1 Mine in New Hampshire, USA.

Rarity

Very rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, spherical aggregates of an intense, reddish-brown color, clearly contrasting with the light background of the host rock. The absence of damage to the delicate, radial clusters is also important. Due to their small size, specimens are typically intended for "micromount" collections. ## Popular Localities Undoubtedly, the most known and valued source of robertsite specimens is its type locality - the Tip Top Mine in South Dakota, USA. Specimens from this location are considered exemplary for this mineral.

Care and storage

## Cleaning Robertsite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its fragility and possible reactivity, avoid washing in water, and especially avoid using ultrasonics. ## What to Avoid Avoid contact with chemicals, acids, and detergents. The mineral is sensitive to scratches and impacts. It should not be heated or exposed to prolonged moisture. ## Storage Collector specimens of robertsite, being rare and delicate, are best stored in enclosed "micromount" boxes, protecting them from dust, light, and mechanical damage. It should be kept away from harder minerals to prevent scratching.