tumchaite

{{infobox mineral | name = Tumchaite | category = Phyllosilicate minerals | boxwidth =330px | boxbgcolor = | image = Tumchaite-163787.jpg | imagesize = 260 | alt = | caption = | formula = | IMAsymbol = Tum | molweight = 462.51 g/mol | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/c | unit cell = a = 9.144 Å, b = 8.818 Å c = 7.537 Å, β = 113.22°; Z = 2 | color = Colorless to white | habit = Prismatic, tabular, massive, granular | twinning = on {100} | cleavage = Perfect on {100} | fracture = Uneven | tenacity = Brittle | mohs = Close to 4.5 | luster = Vitreous | streak = Colorless to white | diaphaneity = Translucent to transparent | gravity = 2.78 | opticalprop = Biaxial (-) | refractive = nα = 1.570, nβ = 1.588, nγ = 1.594 | birefringence = δ = 0.0240 | pleochroism = Colorless to greenish-gray | 2V = 60 (5)° | dispersion = | extinction = | length fast/slow = | fluorescence= None | absorption = | melt = | fusibility = | diagnostic = | solubility = Does not dissolve with dilute HCl | other = | alteration = | references = }}

Tumchaite, , is a colorless to white monoclinic phyllosilicate mineral. It is associated with calcite, dolomite, and pyrite in the late dolomite-calcite carbonatites. It can be transparent to translucent; has a vitreous luster; and has perfect cleavage on {100}. Its hardness is 4.5, between fluorite and apatite. Tumchaite is isotypic with penkvilksite. The structure of the mineral is identified by silicate sheets parallel {100}, formed by alternation of clockwise and counterclockwise growing spiral chains of corner-sharing tetrahedra. Tumchaite is named for the river Tumcha near Vuoriyarvi massif.