Hawthorneite

Chemical formula: BaMgTi⁴⁺₃Cr³⁺₄Fe²⁺₂Fe³⁺₂O₁₉

Hawthorneite is a rare, black or brown mineral with a metallic luster, belonging to the magnetoplumbite group, found in kimberlites.

## Characteristics Hawthorneite is a complex oxide of barium, magnesium, titanium, chromium, and iron, belonging to the hematite supergroup and the hawthorneite subgroup. It occurs as very small, hexagonal crystals with a tabular habit, usually not exceeding 0.25 mm in size. It is an opaque mineral, black or brown in color, with a characteristic metallic luster. ## Physical Properties Its Mohs hardness is 5.5-6. The luster is metallic, and the mineral is completely opaque. Due to its extreme rarity and microscopic crystal sizes, precise density determination is difficult and has not been published. ## Colors and Varieties This mineral occurs in uniform colors – black or brown. No color or commercial varieties are distinguished. ## History and Name The mineral is named in honor of John Barry Hawthorne (born 1931), a geologist from Kimberley, Republic of South Africa, in recognition of his contribution to the understanding of kimberlite geology and mineralogy. The mineral was described and approved by IMA in 1987. ## Uses Hawthorneite has no industrial application. Its significance is purely scientific and collectible, although due to its microscopic size, it is primarily of interest to specialized micromount collectors.

Properties

Mohs hardness
5.5-6
Luster
Metallic
Density
0
Transparency
Opaque
Crystal system
Hexagonal

Diagnostic features

## Identification Amateur identification of hawthorneite is practically impossible. It requires advanced analytical techniques, such as X-ray microanalysis (EDS/WDS), due to its microscopic size and co-occurrence with other similar minerals. Helpful features under the microscope include the hexagonal crystal shape, black color, and metallic luster. ## Distinguishing from Similar Minerals Hawthorneite is visually indistinguishable from other black, metallic minerals of the magnetoplumbite group, such as magnetite, chromite, or yimengite, which often co-occur with it. Definitive differentiation is only possible based on chemical composition analysis. ## Crystal Forms It forms very small, sharp-edged, hexagonal crystals with a tabular habit. These crystals occur as inclusions in other minerals, mainly in diamonds and pyropes.

Geological environment

## Genesis Hawthorneite is an igneous mineral, crystallizing under high pressure and temperature conditions deep within the Earth's mantle. It forms in ultramafic rocks, mainly in kimberlites. ## Mineral Associations This mineral most commonly occurs as an inclusion in diamonds. It also co-occurs with minerals such as pyrope (chromian variety), enstatite, ilmenite, lindsleyite, mathiasite, and chromian spinel. ## Localities The most important and typical locality for hawthorneite is the Bultfontein mine in Kimberley, Republic of South Africa. This is the only confirmed and thoroughly described locality for this mineral worldwide.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, hawthorneite is valued primarily for the mere fact of its confirmed presence in a specimen. The most desirable specimens are those in which hawthorneite crystals are well-formed, have a distinct hexagonal outline, and are visible against a contrasting rock matrix or, which is extremely rare, as inclusions in transparent diamonds. Crystal size, even if measured in fractions of a millimeter, is a key factor increasing value. ## Popular Localities The only source of collectible specimens is the historical type locality – the Bultfontein mine in Kimberley (RSA). Specimens from this location are extremely rare on the market.

Care and storage

## Cleaning Specimens, due to their microscopic nature, usually do not require cleaning. If necessary, compressed air can be used to remove dust. Avoid contact with water and chemicals. ## What to Avoid Avoid contact with acids and other aggressive chemicals. Due to the small size of the crystals, specimens are very susceptible to mechanical damage. There is no detailed data on sensitivity to light or temperature, but standard caution is recommended. ## Storage Hawthorneite specimens should be stored in specialized "micromount" boxes that protect them from dust, moisture, and mechanical damage. Display is only possible under a microscope.

External references

Sources

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