Paracostibite

Chemical formula: Co<sup>₃+</sup>(SbS)<sup>₃-</sup>

Paracostibite is a rare, metallic cobalt and antimony mineral, closely related to costibite.

## Characteristics Paracostibite is a rare cobalt antimony sulfide. It typically occurs as small grains disseminated in rock or as rims around grains of other minerals, such as costibite. Its identification with the naked eye is practically impossible and requires laboratory analysis. It has a metallic luster and a steel-gray to pinkish-gray color. ## Physical Properties This mineral is characterized by high hardness, ranging from 6-7 on the Mohs scale, which is unusual for many sulfides. It has a strong, metallic luster and is opaque. Its density is significant, calculated at 8.25 g/cm³. ## Colors and Varieties Paracostibite is steel-gray to pinkish-gray, often with a violet tint. In reflected light under a microscope, its color is described as gray with a pinkish or violetish hue. No varieties are distinguished. ## History and Name Paracostibite was first described in 1970 by L.J. Cabri, D.C. Harris, and J.M. Stewart. Its name refers to its structural and chemical similarity to costibite (CoSbS), with the prefix "para-" indicating its different, orthorhombic crystal symmetry. It was discovered in copper-nickel deposits in the Red Lake area of Ontario, Canada.

Properties

Mohs hardness: 6-7
Luster: Metallic
Streak: Black
Density: 8.25
Cleavage: Undetermined
Fracture: Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Identification of paracostibite is possible only through advanced laboratory techniques, such as electron microprobe analysis (EDS/WDS) and X-ray diffraction (XRD). In reflected light under a polarizing microscope, it exhibits weak anisotropy. Its hardness is a distinguishing feature among some sulfides. ## Distinguishing from Similar Minerals Paracostibite is almost impossible to distinguish from costibite with the naked eye, with which it often co-occurs. Costibite crystallizes in the isometric system, while paracostibite crystallizes in the orthorhombic system, which is the fundamental differentiating feature, ascertainable only by crystallographic methods. It can also be confused with other metallic sulfides and arsenides, such as safflorite or skutterudite, from which it is distinguished by its chemical composition.

Geological environment

## Genesis Paracostibite is a hydrothermal mineral. It forms in ore veins, most often associated with nickel, cobalt, and copper deposits. It forms over a relatively wide range of temperatures, often as a result of alterations of pre-existing cobalt and antimony minerals. ## Mineral Associations It most commonly co-occurs with costibite, chalcopyrite, cubanite, pentlandite, pyrite, sphalerite, and also with platinum-group minerals (PGM). At its discovery site (Red Lake, Ontario), it was found in association with costibite, chalcopyrite, and pyrite. ## Localities Paracostibite is a very rare mineral, known from only a few localities worldwide. Besides the type locality in Red Lake, Ontario (Canada), its occurrence has been confirmed, among others, in the niccolite-galkhaite deposit in the Khaidarkan mine in Kyrgyzstan and in the Koillismaa layered intrusion in Finland.

Rarity

Very rare

For collectors

## Quality Criteria From a collector's perspective, the value of paracostibite specimens lies not in their aesthetics, but in their rarity and scientific significance. The most desirable specimens are those with analytically confirmed presence of this mineral, originating from a well-documented locality, especially from the type locality (Red Lake, Ontario). A rich association with other rare minerals, such as costibite, increases its value.

Care and storage

## Cleaning Due to its occurrence as microscopic grains within the host rock, paracostibite specimens generally do not require and are not amenable to cleaning. Any attempts at mechanical or chemical cleaning may damage both the mineral itself and the surrounding minerals. ## What to Avoid Avoid contact with strong acids and chemicals that could react with sulfides. Specimens should not be heated. Due to its stability, no special precautions regarding light or humidity are required. ## Storage Specimens containing paracostibite should be stored under stable conditions, in collector's boxes, protected from dust and mechanical damage. The label should precisely indicate the location of the microscopic grains.

Sources

IMA Mineral List (CNMNC)