Mineralogy and Petrography of Phosphate Mineral Associations From the Jocão Pegmatite, Minas Gerais, Brazil

Abstract

The Jocão pegmatite (or Cigana) is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. According to their macroscopic textures, three different kinds of phosphate mineral masses were collected from the dumps of the pegmatite: association I is composed of dendritic triphylite, forming intergrowths with silicate minerals (spessartine garnet or albite); association II forms blocky nodules of triphylite-ferrisicklerite-heterosite; and association III shows exsolution lamellae of ferrisicklerite-heterosite in massive beusite. The primary textures and the Fe/(Fe+Mn) ratios of primary triphylite make it possible to establish the crystallization sequence of the primary phases; the petrogenesis of primary intergrowths between garnet and triphylite of association I is also discussed. In the three associations, these primary minerals are hydrothermally altered and secondary species are produced. In association I, the first hydrothermal alteration event was a weakly oxidizing hydroxylation stage, during which triphylite was only replaced by hureaulite (rarely associated with barbosalite) along its cleavage planes. The final stage affecting association I corresponds to meteoric processes during which ludlamite and then vivianite progressively replaced triphylite. Association II evolved under more oxidizing conditions and the first alteration stage corresponds to the progressive oxidation of triphylite accompanied by Li-leaching, leading to ferrisicklerite and heterosite. The second hydrothermal stage corresponds to a hydroxylation event, and the secondary species depend on the phosphate mineral that they replace: triphylite is only altered to Fe 2+ -Mn 2+ -bearing hydrated species (colorless hureaulite); ferrisicklerite is altered to Fe 2+ -, Mn 2+ -, and Fe 3+ -bearing phosphate minerals such as jahnsite s.l., frondelite s.l., and orange hureaulite; heterosite is replaced by Fe 3+ -bearing species such as ferristrunzite. Consequently, it appears that the secondary phosphate minerals, which crystallize during this second hydrothermal stage, strongly depend on the cations which are locally available in the sample zone. The final stage forms the meteoric species: leucophosphite and phosphosiderite directly replace heterosite, whereas vivianite, a ferrous meteoric species, only appears in the triphylite core. In association III, exsolution lamellae are formed at the expense of a high-temperature homogenous Ca-Li-bearing graftonite-beusite-like phase; when the temperature decreased, Li migrated into triphylite and Ca to the larger M1 site of beusite. During the high-temperature hydrothermal alteration processes, triphylite transforms into ferrisicklerite and heterosite, like in association II; however, beusite is not affected by any transformation process at that stage. During the low temperature hydroxylation stage, ferrisicklerite from the core remains almost unaltered, while beusite is replaced by an intimate mixture of pleochroic Ca-rich hureaulite and tavorite. After this hydroxylati...

Single-crystal neutron diffraction and Mössbauer spectroscopic study of hureaulite, (Mn,Fe)5(PO4)2 (HPO4)2 (H2O)4

“…This strongly fractionated pegmatitic dyke, belonging to the beryl-columbite-phosphates subtype in the classification of Č erný & Ercit (2005), shows a rather simple primary phosphate association given by triphylite þ beusite (e.g. Baijot et al, 2014). According to Baijot et al (2014), three different primary phosphate associations originate several secondary phosphate assemblages containing hureaulite as a common product of alteration.…”

“…Baijot et al, 2014). According to Baijot et al (2014), three different primary phosphate associations originate several secondary phosphate assemblages containing hureaulite as a common product of alteration. The three primary phosphate associations originate three kinds of hureaulite, which are different both in the aspect and chemical composition.…”

“…The three primary phosphate associations originate three kinds of hureaulite, which are different both in the aspect and chemical composition. The specimen of hureaulite used in this study corresponds to the ''hureaulite II'' described by Baijot et al (2014). A description of other associated minerals at the Jocão (Cigana) pegmatite, among which the recently discovered correianevesite, is given in Chukanov et al (2014).…”

Structural Incorporation of As5+into Phosphosiderite by a Strengite/Scorodite-like Arrangement

“…Phosphosiderite (FePO 4 ⋅2H 2 O), which is also known as metastrengite or clinostrengite, is a common weathering product of primary phosphates like heterosite (Fe 0.65 Mn 0.35 PO 4 ), Fe‐bearing hureaulite (Mn 5 (PO 4 ) 2 (PO 3 OH) 2 •4H 2 O) and tripylite (Li(Fe 2+ ,Mn 2+ )PO 4 ) . It occurs frequently in pegmatitic rocks, where it forms by supergene processes together with the polymorph strengite .…”

Secondary blue tourmaline after garnet from elbaite-subtype pegmatites; implications for source and behavior of Ca and Mg in fluids

“…Schreyer et al 1993;Jobin-Bevans and Černý 1998;Gadas et al 2016), beryl (Černý 2002), andalusite (e.g., Ahn and Buseck 1988), and primary Li,Fe,Mn-phosphates (e.g., Dill 2009;Roda-Robles et al 2012;Baijot et al 2014). These primary coarse minerals are locally completely replaced by a fine-grained mineral assemblage or a single mineral.…”