Abstract: Okhotskite, an Mn³⁺-dominant pumpellyite-group mineral, is monoclinic, A2/m, a = 8.887, b = 6.000, c = 19.55 Å, β = 97.08°, Z = 1. The formula is (Ca7.63Na0.17K0.01)Σ7.81(Mn2.752+Mg1.10)Σ3.85(Mn4.503+Al1.87Fe1.613+Ti0.02)Σ8.00Si12.13O39.71(OH)16.29, which simplifies to Ca₈(Mn²⁺,Mg)₄(Mn³⁺,Al,Fe³⁺)₈Si₁₂O_5∂-n(OH)_n. Strong lines in the X-ray powder diffraction pattern are: 4.76(60)(004, 111), 3.87(70)(202), 2.96(100)(115, 300), 2.88(25)(022), 2.72(70)(302), 2.67(45)(3¯04, 3¯11), 2.55(45)(024, 3¯13) and 2.38(45)(304).

Its colour is deep orange with a pale orange streak and vitreous lustre. Transparent. Non-fluorescent. H6 (Mohs). Density (calc.) = 3.40 g/cm³. Optically biaxial negative, 2V(meas) = 46(5)°, 2V(calc) = 46°, dispersion indiscernible. Refractive indices: α = 1.782(5), β = 1.820(5), γ = 1.827(5). Orientation: Y = b, c∧Z = 9–14° in acute angle of β. Pleochroism is distinct: X yellow, Y and Z deep orange. Absorption: X < Y < Z.

This mineral occurs as fine prisms up to 0.2 mm long and forms aggregates in network veinlets cutting hematite ore from the Kokuriki mine in the Tokoro district, eastern Hokkaido, Japan. Associated minerals are: hematite, piemontite, quartz, neotocite, bementite, apatite, Mn-oxides, inesite and rhodochrosite. The mineral is named okhotskite for the Sea of Okhotsk, along which the mine is located.