Morimotoite Identification Guide

What Morimotoite Looks Like

Morimotoite is a very rare member of the garnet group — a titanium-rich, iron-bearing calcium garnet, ideally Ca3(Ti,Fe)2(Si,Fe)3O12. It is black to brownish-black and opaque, with a vitreous to slightly resinous luster. Like all garnets it is isometric, so when crystals form they show the classic rhombic dodecahedron / trapezohedron shapes, though it usually occurs as small grains in skarn rock.

• Color: black to brownish-black
• Luster: vitreous to resinous
• Transparency: opaque
• Habit: isometric crystals (dodecahedra/trapezohedra) and anhedral grains in skarn

Step-by-Step Field ID Checklist

1. Confirm garnet form. Look for isometric, equant crystals (12- or 24-sided) embedded in calc-silicate skarn.
2. Check hardness: Mohs ~7–7.5 — scratches glass and resists a knife.
3. No cleavage. Garnet breaks with conchoidal to uneven fracture (separates it from cleaved black minerals).
4. Note the geologic setting: a black garnet in a Ca-rich skarn with magnetite, calcite, and other silicates raises the possibility of a titanian garnet.
5. Accept the limits. Morimotoite cannot be reliably told from other Ti-garnets by eye — confirmation needs chemical/optical lab analysis.

Key Diagnostic Tests

• Hardness: ~7–7.5.
• Cleavage: none; conchoidal fracture.
• Specific gravity: high (~3.7–3.9, typical of Ti-andradite garnets).
• Crystal system: isometric.
• Definitive ID requires microprobe chemistry (high Ti, characteristic Fe distribution).

Common Look-Alikes and How to Tell Them Apart

• Melanite (titanian andradite): black andradite garnet — chemically very close; melanite has Ti substituting differently, and the two are distinguished only by precise composition. Visually they are nearly identical.
• Schorlomite: another black Ti-rich garnet with even higher Ti; again, lab chemistry is required to separate it from morimotoite.
• Schorl (black tourmaline): forms striated three-sided prisms with no garnet shape — easy to separate by crystal form.
• Magnetite: strongly magnetic and gives a black streak, unlike garnet.
• Andradite/melanite group: all share the look; only quantitative analysis distinguishes the species.

Where Morimotoite Is Found

Morimotoite was described from the Fuka skarn in Okayama Prefecture, Japan, and occurs in calc-silicate skarns where titanium-bearing fluids reacted with carbonate rocks. It is a rare collector and research species, generally found as grains in skarn assemblages with other calcium silicates and oxides.

Forms, Treatments, and Field Notes

Morimotoite is a research-grade rarity, not a jewelry stone, so the realistic goal in the field or cabinet is to recognize that you have a titanian (Ti-rich) andradite-group garnet and then send it for analysis. Its skarn setting is the biggest clue: black isometric garnets grown in calc-silicate rocks with calcite, magnetite, vesuvianite, wollastonite, and diopside should raise the possibility of a Ti-garnet such as morimotoite, melanite, or schorlomite.

Why lab work is required

The garnet group forms continuous chemical solid solutions, and morimotoite, melanite (Ti-andradite), and schorlomite differ mainly in the amount and substitution of titanium and iron. No hardness, color, or luster test separates them, so positive identification relies on electron microprobe chemistry and sometimes optical/Mössbauer study. For the collector, label such specimens as "titanian garnet (morimotoite group)" unless backed by an analysis from a credible source.