Grimmite

Chemical formula: Ni₂+Co₃+₂S₂-₄

Grimmite is an extremely rare nickel-cobalt sulfide, forming microscopic, metallic grains within ultramafic rocks.

## Characteristics Grimmite is a sulfide mineral belonging to the linnaeite group. It occurs as extremely small, individual, rounded grains not exceeding 150 micrometers in size. Observed under a microscope in reflected light, in polished sections, it has a creamy-white color with a pinkish tint and a strong, metallic luster. Due to the microscopic size of the crystals, macroscopic features cannot be observed. ## Physical Properties Due to the microscopic size of the grains, many physical properties of grimmite, such as hardness, density, cleavage, or fracture, have not been determined. The mineral is opaque and exhibits a strong, metallic luster. Its streak is black. ## History and Name The mineral is named in honor of Johann Grimm (1959-2012), a Czech geologist and mineralogist, who first discovered this mineral in 1992. Grimm was a specialist in the mineralogy of ore deposits and nickel-cobalt deposits. The mineral was formally approved by the International Mineralogical Association (IMA) in 2018 (IMA2018-027). ## Uses Grimmite has no industrial or commercial applications. Its significance is purely scientific and as a collector's item, as an extremely rare member of the linnaeite group.

Properties

Luster: Metallic
Streak: Black
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Identification of grimmite is possible only through advanced laboratory techniques, such as ore microscopy (reflected light analysis) and chemical analysis using an electron microprobe (EDS/WDS). It cannot be identified with the naked eye or in field conditions. ## Distinguishing from similar minerals Grimmite is visually almost identical to other minerals of the linnaeite group, such as linnaeite, siegenite, or polydymite, with which it often co-occurs. Differentiation requires precise analysis of the nickel to cobalt ratio in its chemical composition. ## Crystal forms The mineral forms exclusively anhedral (irregular), rounded grains and aggregates of microscopic size, occurring as inclusions in other sulfides or silicates.

Geological environment

## Genesis Grimmite forms as a result of hydrothermal processes within serpentinized ultramafic rocks (peridotites). It crystallizes in paragenesis with other nickel and cobalt sulfides. ## Mineral associations This mineral co-occurs with other sulfides, such as siegenite, polydymite, pentlandite, millerite, chalcopyrite, and pyrite. It is also accompanied by platinum-group minerals (PGM), as well as chromite, magnetite, and serpentine-group minerals. ## Localities The only confirmed occurrence of grimmite in the world (type locality) is the serpentinite quarry in Drahonín, near Tišnov in Moravia, Czech Republic.

Rarity

Extremely rare

For collectors

## Quality criteria As a "micromount" mineral, the collector's value of grimmite depends on the confirmation of its identity through chemical analysis. The most valuable specimens are those where grimmite co-occurs with other rare sulfides, and its presence is analytically documented. Grain size, although always microscopic, is also important – larger grains (above 100 micrometers) are more desirable.

Care and storage

## Cleaning Grimmite specimens occur as microscopic inclusions within the host rock and do not require cleaning. Any attempt at mechanical or chemical cleaning would destroy the mineral. ## What to avoid Avoid contact with chemicals, acids, and ultrasound. The host rock containing grimmite should be protected from moisture to prevent the oxidation of sulfides. ## Storage Specimens should be stored under stable conditions, in a dry place, preferably in a specialized "micromount" box, to protect them from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)