Coldwellite

Chemical formula: Pd<sub>3</sub>Ag<sub>2</sub>S

Coldwellite is a very rare palladium-silver sulfide, found as microscopic grains in igneous rocks.

## Characteristics Coldwellite is a palladium-silver sulfide, occurring as a very rare accessory mineral. It typically forms extremely small, anhedral (irregular) grains, not exceeding a few tens of micrometers in size. It is observed exclusively in polished sections under an electron microscope, where it appears as bright, metallic inclusions within other minerals, mainly copper and iron sulfides. ## Physical Properties Due to the microscopic size of its crystals, most of Coldwellite's physical properties have not been determined using standard methods. Hardness, density, cleavage, and fracture remain unknown. Its luster is metallic, and the mineral is opaque. ## Colors and Varieties In reflected light (under the microscope), Coldwellite has a neutral gray color. It does not exhibit pleochroism or anisotropy. No varieties of this mineral have been distinguished. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2010 (IMA2010-021). Its name, Coldwellite, honors the Coldwell igneous complex in Ontario, Canada, where it was discovered. The first description was provided by a research group led by A. M. McDonald. ## Applications Coldwellite has no commercial or industrial applications. Its significance is purely scientific, serving as an indicator of geochemical processes related to the concentration of platinum-group metals in specific types of igneous intrusions.

Properties

Luster
Metallic
Cleavage
None
Transparency
Opaque
Crystal system
Cubic

Diagnostic features

## Identification Identification of Coldwellite is possible only through advanced laboratory techniques. It requires chemical composition analysis using an electron microprobe (EDS/WDS) on a polished rock section. In reflected light, it is a gray, metallic mineral with low reflectivity compared to other platinum-group metal sulfides. ## Distinguishing from Similar Minerals It can be confused with other microscopic sulfides and noble metal alloys. Final differentiation from, for example, palladseite (Pd₁₇Se₁₅) or wassilite (Pd₃S) requires precise quantitative chemical analysis. ## Crystal Forms Coldwellite occurs exclusively as anhedral (irregular), rounded grains ranging from 5 to a maximum of 50 micrometers in size. It often forms complex intergrowths with bornite, chalcocite, and other silver sulfides.

Geological environment

## Genesis Coldwellite crystallizes in the late stages of magmatic processes within basic and ultrabasic intrusions. Its formation is associated with the differentiation of magma rich in sulfides and platinum-group elements (PGE). It occurs in pegmatitic zones within gabbro. ## Mineral Associations This mineral co-occurs with bornite, chalcocite, digenite, covellite, sphalerite, magnetite, titanite, apatite, epidote, and other silver and platinum-group minerals, such as wassilite and palladseite. ## Localities The only confirmed and thoroughly described occurrence (type locality) is the Coldwell alkaline complex, near Marathon in Ontario, Canada. It was found there in pegmatitic veins cutting gabbro.

Rarity

Extremely rare

For collectors

## Quality Criteria Coldwellite does not occur in the form of collector specimens available on the market. Its value is purely scientific. The only form in which it can be possessed is a fragment of the host rock from the type locality, where the presence of the mineral has been analytically confirmed. The value of such a specimen is symbolic and depends on its scientific value and documentation.

Care and storage

## Cleaning Due to its microscopic occurrence within the host rock, individual specimens of Coldwellite are not isolated. Cleaning pertains to the entire rock and is usually limited to removing dust with compressed air or a soft brush. ## What to Avoid Rock specimens containing Coldwellite should be protected from aggressive chemicals that could react with sulfides. Acids and strong oxidizing agents should be avoided. ## Storage Storage under stable room conditions, in a dry place, is entirely sufficient. Specimens most often come in the form of polished sections or rock fragments, which are best kept in collector boxes, protected from dust and mechanical damage.

Sources

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