Mertieite

Chemical formula: Pd<sub>8</sub>Sb<sub>2.5</sub>As<sub>0.5</sub>

Mertieite-II is a rare palladium arsenoantimonide, forming tiny, metallic grains in ultramafic rocks.

## Characteristics Mertieite-II is a rare mineral from the palladium arsenoantimonide group. It occurs as small, rounded or irregular grains, usually not exceeding 1.5 mm in size. It is most often intergrown with other platinum-group minerals (PGM) or sulfides. Due to its chemical composition, it is a heavy and opaque mineral with a metallic luster. ## Physical Properties This mineral is characterized by high hardness, exceeding 6 on the Mohs scale, which is unusual for most native metals and their alloys. It has a metallic luster and is opaque. Its density is very high, resulting from the high palladium content. ## Colors and Varieties The color of mertieite-II in reflected light is creamy white with a pinkish tint. No color or commercial varieties are distinguished. ## History and Name The mineral was first described in 1973 by G.A. Desborough, J.J. Finney, and B.F. Leonard based on material from the Goodnews Bay platinum mine in Alaska, USA. The name honors John Beaver Mertie Jr. (1888-1980), a geologist with the United States Geological Survey, for his contributions to the study of platinum deposits in Alaska. Initially described as mertieite, it was later, in 1975, redefined and renamed mertieite-II due to the discovery of the polymorphic variety mertieite-I. ## Applications Due to its extreme rarity and microscopic size, mertieite-II has no industrial applications. It is solely an object of scientific and collecting interest within systematic collections.

Properties

Mohs hardness
>6
Luster
Metallic
Streak
Black
Density
11.12
Cleavage
None
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Identification of mertieite-II is only possible using advanced laboratory methods, such as chemical composition analysis by electron microprobe (EDS/WDS) and X-ray diffraction (XRD) analysis. Visually, in a polished section under a microscope, it can be recognized by its creamy white color with a pinkish tint and high reflectivity. ## Distinguishing from Similar Minerals It can be confused with other platinum-group minerals (PGM) such as sperrylite, native platinum, stibiopalladinite, or mertieite-I. Differentiation requires specialized studies, as all these minerals have a similar metallic appearance and occur in the same parageneses. Mertieite-II is distinguished by its unique combination of palladium, antimony, and arsenic in specific proportions and its trigonal crystal structure. ## Crystal Forms This mineral forms exclusively anhedral (unformed) grains, often with rounded contours. Well-formed crystals are not observed.

Geological environment

## Genesis Mertieite-II is a mineral of magmatic origin. It crystallizes in the late stages of processes occurring in ultramafic magmatic intrusions, rich in chromite. It forms as a result of the segregation of platinum-group metals from sulfidic magma. ## Mineral Associations It co-occurs with other platinum-group minerals and sulfides. Its most common associated minerals are: chromite, sperrylite, native platinum, laurite, irarsite, braggite, chalcopyrite, pentlandite, and pyrrhotite. ## Localities The most important and classic locality, from which the type material originates, is the platinum deposits in the Goodnews Bay area in Alaska, USA. This mineral has also been identified in the Bushveld Complex in South Africa, the Stillwater Complex in Montana (USA), in nickel and copper deposits near Norilsk and on the Kola Peninsula in Russia, as well as in Finland and Brazil.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a mertieite-II specimen is primarily assessed based on confirmation of its identity through chemical analysis. Since it occurs as microscopic grains, collector specimens are usually rock fragments or polished sections with a marked and identified grain. Specimens with richer concentrations or larger (though still sub-millimeter) grains are more highly valued, as are those from the type locality (Goodnews Bay, Alaska). ## Popular Localities The most sought-after by specialized collectors are specimens from the type locality in Alaska and from the Bushveld Complex in South Africa, from which many classic platinum-group mineral specimens originate.

Care and storage

## Cleaning Specimens containing mertieite-II, which are typically microscopic grains intergrown in the host rock, are generally not cleaned. Any mechanical or chemical intervention can damage or remove the delicate mineral grains. If necessary, dust can be removed with a gentle stream of compressed air. ## What to Avoid Avoid contact with chemicals, acids, and ultrasonic cleaners. Do not expose the specimen to sudden temperature changes. Mechanical cleaning, even with a soft brush, is not recommended due to the risk of dislodging small grains. ## Storage Specimens with mertieite-II are best stored under stable conditions, in closed "micromount" boxes, which protect against dust and mechanical damage. Avoid moisture and direct sunlight.

Sources

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