Yoshimuraite, a new barium-titanium-manganese complex silicate mineral, was found in an alkali-pegmatitic facies bordering the bedded manganese deposit of the Misago ore body of the Noda-Tamagawa mine. Yoshimuraite occurs as dark-brownish platy to tabular crystals, or sometimes in the stellate groups embedded in coarse aggregates of barium-bearing potashfeldspar, quartz, richterite, urbanite, rhodonite etc.
It resembles lamprophyllite and astrophyllite which usually occur in alkali-pegmatites. It is orange brown in colour, brittle and has a perfect cleavage parallel to {010} and distinct cleavages on {10 _?? } and {101}. Polysynthetic twinning on {010} is common. Hardness, 41/2; Sp. gr., 4.13 (obs.) and 4.21 (calc.). Triclinic, with a₀=7.00, b₀=14.71, c₀=5.39Å, α=93.5°, β=90.2°, γ=95.3°, and a:b:c=0.476:1:0.366. Space group, P _?? . Biaxial positive with α=1.763, β=1.777, γ=1.785, (+)2V=85°-90° ρ⟩υ, and absorption X⟨Y ?? Z. Optical orientation, γ′ nearly parallel to c-axis, α′ nearly parallel to a-axis. Pleochroism, α bright yellow, β orange-brown, γ brown.
Chemical analysis by J. Ito gave SiO₂ 18.25, TiO₂ 10.00, Fe₂O₃ 1.32, FeO 1.47, MnO 17.64, MgO 0.56, ZnO 0.50, BaO 33.51, SrO 4.62, Na₂O 0.16, K₂O 0.03, P₂O₅ 3.98, SO₃ 5.40, Cl 0.41, H₂O(±) 2.34, less for Cl 0.09, total 100.10%. The results indicate a formula of approximately X₂Y₂ (Ti, Fe) O (Si₂O₇) ?? (P, S)O₄ ?? (OH) or X₂Y₂ (Ti, Fe) (SiO₄)₂ ?? (P, S)O₄ ?? (OH), where X=Ba, Sr and Na, Y=Mn, Fe, Mg and Zn.
The mineral is named after Professor Toyofumi Yoshimura who has contributed much to the study of manganese minerals in Japan.