Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Jaromír Tvrdý, Luboš Vritiška, Jan Filip, Zdeněk Dolníček, Radek Škoda, Martin Petr, Malíková Radana

Manganrockbridgeite from the Jocão claim, Conselheiro Pena, Minas Gerais, Brazil: chemistry, spectroscopic and structural data

Journal of Geosciences, volume 69 (2024), issue 2, 87 - 98

DOI: http://doi.org/10.3190/jgeosci.387

The world’s second occurrence of the recently described manganrockbridgeite was found in a mineral assemblage in the Jocão (Cigana) pegmatite, Conselheiro Pena district, Minas Gerais (Brazil). Manganrockbridgeite forms green botryoidal-crystalline crusts with a velvety surface covering the walls of leached cavities in triphylite replaced by correianevesite and Mn-rich vivianite. The outer zone is significantly richer in Mn than the inner zone where the manganrockbridgeite passes to frondelite. Other associated minerals are hureaulite and kenngottite-like phosphate. Empirical formulae based on EPMA, Mössbauer and XPS spectroscopy are (Mn_1.36Fe²⁺_0.46Fe³⁺_0.09Ca_0.03Mg_0.01  0.05)_Σ2.00 Fe³⁺_2.95(PO₄)_3.00(OH)_3.84(H₂O)_1.16 for the outer zone and (Mn_1.03Fe²⁺_0.49Fe³⁺_0.29Ca_0.03Mg_0.02  0.14)_Σ2.00 Fe³⁺_2.94(PO₄)_3.00(OH)_3.81(H₂O)_1.19 for the inner zone of aggregates. Raman spectra and assignment of individual decomposed bands to vibrational modes show a good agreement with data published for frondelite from the Jocão locality. By powder diffraction data the studied manganrockbridgeite corresponds to the type material from Hagendorf-Süd, Germany. Unit-cell parameters for space group P2₁/m are a = 5.195(2) Å, b = 16.936(4) Å, c = 7.452(2) Å, β = 110.25(3)° and V = 615.1(4) Å³.