Morimotoite

Chemical formula: Ca₃Ti₄+Fe₂+Si₃O₁₂

Morimotoite is a rare, black mineral from the garnet group, a calcium, titanium, and iron silicate, found in skarns.

## Characteristics Morimotoite is a rare mineral belonging to the garnet group. It typically forms small, well-formed crystals with a regular, isometric habit, often in the form of a rhombic dodecahedron or a deltoidal icositetrahedron, reaching up to 2 mm in size. It also occurs as granular aggregates. It is an opaque mineral with a black color and a strong, almost metallic luster. ## Physical Properties Morimotoite is characterized by high hardness, approximately 7.5 on the Mohs scale. Its luster is described as vitreous to almost metallic. The mineral's density is significant, around 3.75 g/cm³. ## Colors and Varieties This mineral is monochromatic – it occurs exclusively in black. No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name honors Nobuo Morimoto (1925-2010), a Japanese mineralogist and crystallographer from Kyoto and Osaka Universities, for his contributions to the study of pyroxene group minerals. The mineral was first described in 1992 after being found in the Fuka mine in Japan. ## Uses Morimotoite has no industrial applications. Its significance is purely scientific and collectible due to its rarity and membership in the important garnet group.

Properties

Mohs hardness: 7.5
Luster: Vitreous to metallic
Streak: White
Density: 3.75
Cleavage: None
Fracture: Uneven to conchoidal
Transparency: Opaque
Crystal system: Isometric

Diagnostic features

## Identification Morimotoite is identified by its black color, strong, almost metallic luster, and the characteristic regular crystal forms of garnets (dodecahedra, icositetrahedra). High hardness (approx. 7.5) is also an important diagnostic feature. However, definitive identification requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Morimotoite can be confused with other black garnets, such as schorlomite or melanite (a variety of andradite). Distinguishing them visually is practically impossible and requires specialized tests to determine their chemical composition, especially the titanium and iron content. ## Crystal Forms Morimotoite crystals are typical of the garnet group and belong to the isometric system. They most often adopt the form of well-developed rhombic dodecahedra {110} and deltoidal icositetrahedra {211}. They occur as single crystals or in granular aggregates within the host rock.

Geological environment

## Genesis Morimotoite is a high-temperature mineral formed during contact metamorphism. It forms in skarns, which are rocks created at the contact between magmatic intrusions (e.g., gabbro) and calcium carbonate-rich sedimentary rocks, such as limestones. It requires an environment rich in calcium, silicon, iron, and titanium. ## Mineral Associations This mineral coexists with other minerals typical of skarns, such as vesuvianite, wollastonite, monticellite, gehlenite, as well as with other garnets (e.g., andradite), calcite, and spinel. ## Localities The most important and classic locality (type locality) for morimotoite is the Fuka mine, Okayama Prefecture, Japan. Its occurrence has also been confirmed in several other locations worldwide, including the Ice River alkaline complex in British Columbia (Canada), Arendal in Norway, and the Latium region in Italy.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp-edged crystals and a strong luster are most valued by collectors. The absence of mechanical damage and attractive placement of crystals on a contrasting host rock (matrix), for example, on white calcite, are important. Crystal size is a key factor – specimens with crystals exceeding 1-2 mm are already considered very good. ## Popular Localities By far the most desired and commonly found specimens in collections come from the type locality – the Fuka mine in Japan. Specimens from other localities, such as Ice River in Canada, are much rarer on the collector's market.

Care and storage

## Cleaning Morimotoite specimens should only be cleaned mechanically, using a soft, dry brush to remove dust. Due to its chemical stability, it can be carefully rinsed with distilled water and then thoroughly dried. Ultrasonic cleaners should be avoided, as they can damage delicate crystals or their intergrowths with other minerals. ## What to Avoid Avoid contact with strong acids and other aggressive chemicals. Despite its high hardness, the mineral is brittle and should be protected from impacts and falls. It does not require special protection from light or humidity under typical collecting conditions. ## Storage Morimotoite specimens are best stored in separate, padded display boxes to prevent scratches and chips from other minerals. It can be safely displayed in closed display cases, protecting it from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)