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Menezesite, the first natural heteropolyniobate, from Cajati, São Paulo, Brazil: Description and crystal structure

¹ Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-080, São Paulo, SP, Brazil
² Departamento de Ciências Exatas, Universidade Federal de Alfenas, Rua Gabriel Monteiro da Silva, 714, 37130-000, Alfenas, MG, Brazil
³ Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970, São Carlos, SP, Brazil

Correspondence: ^* E-mail: datencio{at}usp.br

Menezesite, ideally Ba₂MgZr₄(BaNb₁₂O₄₂)·12H₂O, occurs as a vug mineral in the contact zone between dolomite carbonatite and "jacupirangite" (=a pyroxenite) at the Jacupiranga mine, in Cajati county, São Paulo state, Brazil, associated with dolomite, calcite, magnetite, clinohumite, phlogopite, ancylite-(Ce), strontianite, pyrite, and tochilinite. This is also the type locality for quintinite-2H. The mineral forms rhombododecahedra up to 1 mm, isolated or in aggregates. Menezesite is transparent and displays a vitreous luster; it is reddish brown with a white streak. It is non-fluorescent. Mohs hardness is about 4. Calculated density derived from the empirical formula is 4.181 g/cm³. It is isotropic, n_meas > 1.93(1) (white light); n_calc = 2.034. Menezesite exhibits weak anomalous birefringence. The empirical formula is (Ba_1.47K_0.53Ca_0.31Ce_0.17Nd_0.10Na_0.06La_0.02)_2.66(Mg_0.94Mn_{0. 23}Fe_0.23Al_0.03)_1.43 (Zr_2.75Ti_0.96Th_0.29)_4.00 [(Ba_0.72Th_0.26U_0.02)_1.00(Nb_9.23Ti_2.29Ta_0.36Si_0.12)_12.00O₄₂]·12H₂O. The mineral is cubic, space group Im (204), a = 13.017(1) Å, V = 2206(1) ų, Z = 2. Menezesite is isostructural with the synthetic compound Mg₇[MgW₁₂O₄₂](OH)₄·8H₂O. The mineral was named in honor of Luiz Alberto Dias Menezes Filho (born 1950), mining engineer, mineral collector and merchant. Both the description and the name were approved by the CNMMN-IMA (Nomenclature Proposal 2005-023). Menezesite is the first natural heteropolyniobate. Heteropolyanions have been employed in a range of applications that include virus-binding inorganic drugs (including the AIDs virus), homogeneous and heterogeneous catalysts, electro-optic and electrochromic materials, metal and protein binding, and as building blocks for nanostructuring of materials.

Key Words: Menezesite • new mineral • polyoxometalates • heteropolyniobate • heteropolyanions • crystal structure • chemical composition • Jacupiranga mine • Cajati • Brazil

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