Pararobertsite

Chemical formula: Ca<sub>2</sub>Mn<sup>3+</sup><sub>3</sub>O<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>·3H<sub>2</sub>O

Pararobertsite is a rare phosphate mineral, forming microscopic, dark reddish-brown crystals with a strong, almost metallic luster.

## Characteristics Pararobertsite is a hydrated calcium and manganese phosphate, belonging to the mitridatite group. It occurs as very small, tabular or bladed crystals, rarely exceeding 1 mm in length. Most often, it forms radial aggregates, rosettes, or spherical clusters. Its crystals are characterized by an intense, dark reddish-brown to almost black color and an exceptionally strong, submetallic or vitreous luster, which gives them a vivid appearance despite their small size. ## Physical Properties This mineral is relatively soft, with a hardness of about 3.5 on the Mohs scale. It is brittle, and its crystals are typically translucent at the edges to opaque. Luster is one of its most characteristic features – described as submetallic to vitreous, very strong. ## Colors and Varieties Pararobertsite occurs in a uniform range of colors, from dark reddish-brown, through brownish-black, to black. No color or commercial varieties are distinguished. ## History and Name The mineral's name refers to its close structural relationship with robertsite. It was approved as a new mineral species by the International Mineralogical Association (IMA) in 1987. It was described based on specimens from the Tip Top mine in Custer County, South Dakota, USA. ## Applications Pararobertsite has no industrial applications. It is solely a mineral of scientific and collector's interest, sought after by specialists and collectors of rare minerals (so-called microminerals).

Properties

Mohs hardness
3.5
Luster
Submetallic to vitreous
Streak
Brown
Density
3.25
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Pararobertsite can be identified by its characteristic radial aggregates composed of tiny, bladed crystals. Key features include its intense, dark reddish-brown color, very strong, almost metallic luster, and its association with other phosphates in granitic pegmatites. ## Distinguishing from Similar Minerals It can be confused with robertsite, from which it is virtually impossible to distinguish without advanced chemical analyses (XRD, EDS). It differs from it by subtle structural features. It may also resemble other dark phosphate minerals from the mitridatite group, but its unique luster and crystal habit are helpful in identification. ## Crystal Forms Crystals are tabular or bladed, often flattened. They typically form radial, stellate, or spherical aggregates and rosettes. They rarely occur as single, well-formed crystals.

Geological environment

## Genesis Pararobertsite is a secondary mineral, formed in complex granitic pegmatites. It forms as a result of hydrothermal alteration of primary phosphates, such as triphylite or lithiophilite, in manganese-rich zones. ## Mineral Associations It most commonly co-occurs with other phosphate minerals, such as robertsite, jahnsite-(CaMnMn), rockbridgeite, hureaulite, strengite, leucophosphite, as well as with quartz, muscovite, and goethite. ## Localities The most important and classic locality, from which the best specimens originate, is the Tip Top mine in Custer County, South Dakota, USA. This mineral has also been identified in several other localities worldwide, including the Palermo No. 1 Mine in New Hampshire (USA) and the Foote Lithium Co. mine in North Carolina (USA).

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens are those with well-formed, sharp rosettes or radial aggregates with a strong luster. Contrast with a light matrix (e.g., quartz or muscovite) is important, as it highlights the dark color of the crystals. The size of the aggregates, though rarely exceeding a few millimeters, also affects value. Specimens without mechanical damage are much more highly rated. ## Popular Localities By far the most sought-after and valued specimens come from the type locality – the Tip Top mine in South Dakota, USA. These are considered the global standard for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using only a soft brush to remove dust. Due to the brittleness and small size of the crystals, wet cleaning and ultrasonic cleaners should be avoided, as they could damage them. ## What to Avoid Avoid contact with chemicals, especially acids, which can damage the mineral. The crystals are brittle and susceptible to mechanical damage. Do not expose specimens to sudden temperature changes. ## Storage It is recommended to store pararobertsite specimens in enclosed "micromount" boxes, which protect them from dust, moisture, and mechanical damage. This is the best way to safely display and store such small and delicate crystals.

Sources

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