A new mineral tatarinovite, ideally Ca ₃ Al(SO ₄ )[B(OH) ₄ ](OH) ₆ ·12H ₂ O, has been found in cavities of rhodingites at the Bazhenovskoe chrysotile asbestos deposit, the Middle Urals, Russia. It occurs (1) as colorless, with vitreous luster, dipyramidal crystals up to 1 mm across in cavities within massive diopside, in association with xonotlite, clinochlore, pectolite and calcite, and (2) as white granular aggregates, up to 5 mm in size, on grossular with pectolite, diopside, calcite and xonotlite. Mohs' hardness is 3; perfect cleavage on (100) is observed. D _meas 1.79(1), D _calc 1.777 g/cm ³ . Tatarinovite is optically uniaxial (+), w = 1.475(2), e = 1.496(2). The IR spectrum contains characteristic bands of SO ₄ ^2- , CO ₃ ^2- , B(OH) ₄ ^- , B(OH) ₃ , Al(OH) ₆ ^3- , Si(OH) ₆ ^2- , OH ^- , and H ₂ O. Chemical composition of tatarinovite (wt %; ICP-AES; H ₂ O and CO ₂ determined by selective sorption of gaseous products of annealing) is: CaO 27.40, B ₂ O ₃ 4.06, A1 ₂ O ₃ 6.34, Fe ₂ O ₃ 0.03, SiO ₂ 2.43, SO ₃ 8.48, CO ₂ 4.2, H ₂ O 46.1, total 99.04. The empirical formula (calculated on the basis of 3 Ca apfu) is: H _31.41 Ca _3.00 (Al _0.76 Si _0.25 ) _∑1.01 ∙ (B _0.72 S _0.65 C _0.59 ) _S1.96 O _24.55 . Tatarinovite is hexagonal, space group P6 ₃ , a = 11.1110(4) Å, c = = 10.6294(6) Å, V = 1136.44(9) Å ³ , Z = 2. Its crystal-chemical formula is: Ca ₃ (Al _0.70 Si _0.30 )∙ {[SO ₄ ] _0.34 [B(OH) ₄ ] _0.33 [CO ₃ ] _0.24 }{[SO ₄ ] _0.30 [B(OH) ₄ ] _0.34 [CO ₃ ] _0.30 [B(OH) ₃ ] _0.06 }(OH ₅ ∙ ₇₃ O _0.27 )∙12H ₂ O. The strongest reflections of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 9.63 (100) (100), 5.556 (30) (110), 4.654 (14) (102), 3.841 (21) (112), 3.441 (12) (211), 2.746 (10) (302), 2.538 (12) (213). Tatarinovite was named in memory of the Russian geologist and petrologist Pavel Mikhai-lovich Tatarinov (1895-1976), a well-known specialist in deposits of chrysotile asbestos. Type specimens are deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.