Hydroxylbenyacarite, (H2O)2Mn2(Ti2Fe)(PO4)4[O(OH)](H2O)10⋅4H2O, a new paulkerrite-group mineral, from the El Criollo mine, Cordoba Province, Argentina

Abstract

AbstractHydroxylbenyacarite, (H2O)2Mn2(Ti2Fe)(PO4)4[O(OH)](H2O)10⋅4H2O, is a new paulkerrite-group mineral from the El Criollo mine, Cordoba Province, Argentina (IMA2023–079). It was found in specimens of altered triplite, in association with bermanite, phosphosiderite, quartz, strengite and manganese oxides.Hydroxylbenyacarite occurs as light greenish-yellow rhombic tablets with dimensions of typically 20 to 50 μm, occasionally to 400 μm. The crystals are flattened on {010}, slightly elongated on [001] and bounded by the {111} and {010} forms. The calculated density is 2.32 g cm–3. Optically, hydroxylbenyacarite crystals are biaxial (+), with α = 1.608(3), β = 1.624(3), γ = 1.642(3) (measured in white light) and 2V(meas.) = 88(2)°. The calculated 2V is 87.5°. The empirical formula is Ca0.06A[K0.46(H2O)0.88□0.66]Σ2.00M1(Mn1.52Mg0.02Fe2+0.35□0.11)Σ2.00M2+M3(Fe3+1.21Al0.02Ti1.77)Σ3.00(PO4)4X[F0.16(OH)0.70O1.14]Σ2.00(H2O)10⋅3.77H2O.The average crystal structure for hydroxylbenyacarite has space group Pbca and unit cell parameters a = 10.5500(3) Å, b =20.7248(5) Å, c = 12.5023(3) Å, V = 2733.58(12) Å3 and Z = 4. It was refined using single-crystal data to wRobs = 0.074 for 2611 reflections with I > 3σ(I). The crystal structure contains corner-connected linear trimers of Ti-centred octahedra that share corners with PO4 tetrahedra to form 10-member rings parallel to (010). K+ cations and water molecules are located in interstitial sites within the rings. Additional corner-sharing of the PO4 tetrahedra with MnO2(H2O)4 octahedra occurs along [010] to complete the 3D framework structure. A new eight-coordinated interstitial site, previously unreported for paulkerrite-group minerals, is occupied by Ca2+ cations. Weak diffuse diffraction spots in reconstructed precession images for hydroxylbenyacarite violate the a and b glide plane extinctions for Pbca and are consistent with local, unit-cell-scale regions of monoclinic, P21/c structure, in which ordering of the interstitial K+ and Ca2+ cations occurs.