Horomanite

Chemical formula: Fe₆Ni₃S₈

Extremely rare iron nickel sulfide, crucial for understanding processes occurring in the Earth's mantle.

## Characteristics Horomanite is an exceptionally rare sulfide mineral, occurring as tiny grains embedded in other minerals, usually not exceeding 100 micrometers in size. When observed under a microscope in reflected light, it ranges in color from creamy to light brown with a pinkish hue. It is opaque and exhibits a metallic luster. ## Physical Properties Due to the microscopic size of its crystals, many physical properties such as hardness, density, or fracture have not been precisely determined. The mineral is anisotropic, which is visible under a polarizing microscope. ## History and Name The name horomanite comes from its discovery locality – the Horoman peridotite massif in the Hidaka region of Hokkaido, Japan. The mineral was first described in 1998 by a group of Japanese scientists: Kazuhito Ozawa, Masaaki Shimizu, and Tokuhei Tagai. It was approved by the International Mineralogical Association (IMA) under number IMA1997-010.

Properties

Luster: Metallic
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Identification of horomanite is possible only using advanced laboratory techniques. It requires analysis under a reflected light microscope, where its characteristic color and anisotropy are visible. Final confirmation of its identity is obtained using an electron microprobe (chemical composition analysis) and X-ray diffraction methods. ## Distinguishing from Similar Minerals Under the microscope, horomanite can be confused with other opaque sulfides, such as pentlandite, pyrrhotite, or troilite, which often coexist with it. Differentiation is based on subtle differences in color, optical properties (pleochroism, anisotropy), and, above all, precise chemical composition analysis. ## Crystal Forms Horomanite forms exclusively anhedral (irregular) grains, embedded in other minerals. It has not been observed in the form of well-developed crystals.

Geological environment

## Genesis Horomanite is a magmatic mineral, crystallizing under high pressure and temperature conditions typical of the Earth's upper mantle. It forms in ultramafic rocks, such as peridotites, as a result of sulfide melt segregation processes from silicate magma. ## Mineral Associations This mineral occurs in paragenesis with other sulfides and rock-forming minerals. It is most often accompanied by: pentlandite, troilite, djerfisherite, as well as olivine, pyrope, diopside, and enstatite. ## Localities The only confirmed and described occurrence of horomanite in the world is its type locality – the Horoman peridotite massif, located in the Hidaka Mountains on Hokkaido Island, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria Horomanite is not available on the collector's market in the form of macroscopic specimens. Its value is purely scientific. The only form in which it can be "possessed" are specialized research preparations (polished sections) found in university and geological institute collections. The value of such a preparation depends on the quality of its execution and the clarity and size of horomanite grains visible under the microscope.

Care and storage

## Cleaning Due to its extreme rarity and microscopic nature of occurrence, horomanite specimens are practically not subject to cleaning. They occur as inclusions in the host rock or as specialized microscopic preparations (polished sections), which only require protection from dust. ## What to Avoid Contact with any chemicals, ultrasonics, and abrasion should be avoided, as this could irreversibly destroy the microscopic mineral grains or damage the polished surface of the section. ## Storage Specimens containing horomanite, most often in the form of thin sections or polished sections, should be stored in specialized boxes for microscopic preparations, in a dry place, away from dust and the risk of mechanical damage.

Sources

IMA Mineral List (CNMNC)