Big and outstanding transparent very pale brown crystal group of the very rare rubidium mineral masutomilite from one of the few known localities worldwide: Hirukawa, Nakatsugawa City, Honshu Island, Japan. Masutomilite is a Rubidium (Rb) mineral. Rb are extremely rare chemical elements in minerals. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Masutomilite
Masutomilite is an IMA-recognized mineral species in the silicate class, with the chemical formula KLiAlMn²⁺(Si₃Al)O₁₀(F,OH)₂.
It crystallizes in the monoclinic crystal system (space group C2). It is a member of the Mica structural group.
Key properties: Mohs hardness 2.5; specific gravity 2.9–2.94; optically biaxial.
IMA symbol Msu, IMA status: approved, named in 1974.
Found in a druse in a granite pegmatite, as the core of a crystal zoned with manganoan zinnwaldite (Tanakamiyama, Japan); as reaction rims coating spessartine-rich garnets in manganese-rich parts of a lithium pegmatite (Ctidru·zice.
It is found associated with Zinnwaldite, topaz, tourmaline, albite, quartz, cassiterite (Tanakamiyama, Japan); manganoan garnet, manganoan elbaite (Ctidru·zice, Czech Republic).
The name refers to Dr. Kazunosuke Masutomi, Japanese amateur mineralogist.