Magnesiochromite

Chemical formula: MgCr<sup>3+</sup><sub>2</sub>O<sub>4</sub>

Magnesiochromite is a rare mineral from the spinel group, an oxide of magnesium and chromium, occurring as small, black, regular crystals.

## Characteristics Magnesiochromite is a mineral belonging to the spinel group, being the magnesium analogue of chromite and the chromium analogue of spinel sensu stricto. It forms continuous solid solutions with them. It usually occurs as small, well-formed, octahedral crystals, which rarely exceed a few millimeters in size. It is also found as rounded grains in rocks or as a component of placer sands. ## Physical Properties This mineral is characterized by significant hardness, reaching 7.5 on the Mohs scale, which makes it resistant to scratching. It has a vitreous luster, and a greasy luster on fracture surfaces. It is an opaque mineral, and only on very thin edges is it translucent, revealing a reddish-brown color. Its density ranges from 4.23 to 4.42 g/cm³. ## Colors and Varieties Typical magnesiochromite is black to brownish-black. Due to the formation of solid solutions with other spinels, its chemical composition and, consequently, some physical properties may vary slightly depending on the iron or aluminum content. However, no named commercial or color varieties are distinguished. ## History and Name The mineral's name, given in 1868 by George J. Brush, directly refers to its chemical composition – the content of magnesium (magnesio) and chromium (chromite). This is a reference to its relationship with chromite, where iron is replaced by magnesium. ## Applications Due to its rarity and small crystal size, magnesiochromite has no industrial significance. It is primarily an object of interest for collectors specializing in minerals from the spinel group and for systematic mineralogists and petrologists, for whom it is an important indicator mineral of rock formation conditions.

Properties

Mohs hardness
7.5
Luster
Vitreous, greasy
Streak
Reddish-brown
Density
4.23 - 4.42
Cleavage
None
Fracture
Uneven, conchoidal
Transparency
Opaque, translucent on thin edges
Crystal system
Cubic

Diagnostic features

## Identification Key diagnostic features of magnesiochromite are its black color, high hardness (7.5 on the Mohs scale), regular, octahedral crystal form, and reddish-brown streak. Its occurrence in ultramafic rocks (peridotites, dunites) or serpentinites is also a strong indication. ## Distinguishing from similar minerals It can be confused with other black spinels, such as chromite, hercynite, or magnetite. - **Chromite** is practically impossible to distinguish without chemical analysis, although it often has a slightly lower luster and a more brownish streak. - **Magnetite** is strongly magnetic, whereas magnesiochromite does not exhibit magnetism. - **Spinel (pleonaste)** can be black, but usually has a lower density and often occurs in metamorphic rocks, not ultramafic ones. Final distinction within the spinel group requires advanced methods, such as X-ray microanalysis (EDS). ## Crystal forms It most often forms small, perfectly developed crystals in the shape of an octahedron. Less commonly, rhombic dodecahedron forms or combinations of these forms are found. It also occurs as irregular or rounded grains in the rock mass.

Geological environment

## Genesis It is a mineral typical of ultramafic igneous rocks, such as peridotites, dunites, and kimberlites, where it crystallizes from magma at high temperatures as one of the first minerals. It also forms in contact metamorphism processes at the contact of magmatic intrusions with dolomites. As a mineral highly resistant to weathering, it accumulates in clastic deposits (sands and gravels) derived from the erosion of parent rocks. ## Mineral associations It most commonly co-occurs with olivine (peridot), pyroxenes (enstatite, diopside), serpentine, chromite, magnetite, brucite, and calcite. ## Localities Significant, though rarely abundant, occurrences of magnesiochromite have been reported in many places around the world. The most important include: - **Finland:** chromium deposits in Kemi and the Outokumpu area. - **Norway:** Feragen region. - **United Kingdom:** Unst island in Shetland, Scotland. - **France:** deposits in the Alps. - **Cuba:** Camagüey province. - **USA:** Lancaster County in Pennsylvania and San Benito County in California. - **Canada:** asbestos deposits in Thetford Mines, Quebec. - **Pakistan:** Kohistan region.

Rarity

Rare

For collectors

## Quality criteria The most valued by collectors are specimens with sharp, well-formed, lustrous crystals in the shape of a perfect octahedron. The attractiveness is enhanced by the contrasting placement of black crystals on a lighter matrix rock, for example, on white calcite or green serpentinite. Crystal size is a key factor – specimens with crystals exceeding 5 mm are already considered very valuable. ## Popular localities Although this mineral occurs in many places, specimens of collector's significance come from a limited number of localities. Those from Pakistan (Kohistan region) and from some localities in Norway and Finland, where well-formed microcrystals have been found, are particularly sought after.

Care and storage

## Cleaning Magnesiochromite specimens are hard and chemically resistant, which facilitates their cleaning. They can be washed in lukewarm water with a mild detergent and a soft brush. After washing, they should be thoroughly rinsed, preferably with distilled water, and left to dry completely. ## What to avoid Avoid contact with strong acids, although the mineral is relatively resistant to them. It does not require special protection from sunlight or humidity. Despite its high hardness, it should be protected from strong impacts that could cause cracking or chipping. ## Storage Specimens, especially those with well-formed crystals, are best stored in separate display boxes to avoid abrasion and mechanical damage from contact with other minerals. They can be safely displayed under standard lighting and temperature conditions.

External references

Sources

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