Bulgarian Chemical Communications, Volume 50, Special Issue J (pp. 270–280) 2018 Structural and chemical evolution of mineral forms of tungsten in the oxidation zone of the Grantcharitza deposit (Western Rhodopes, Bulgaria) M. P. Tarassov*, E. D. Tarassova Institute of Mineralogy and Crystallography “Acad. Ivan Kostov”, Bulgarian Academy of Sciences, Acad. Georgy Bonchev Str., bl. 107, 1113 Sofia, Bulgaria Received March, 2018; Revised June, 2018 Two principal trends of structural and chemical evolution of mineral forms of W are distinguished in the oxidation zone of the Grantcharitza tungsten deposit. The first trend concerns the processes that occur when the overall pH of supergene solutions decreases. The first group of processes include: (1) dissolution of scheelite, CaWO4, accompanied by the formation of polytungstate ions in the solution at pH ~6–4; (2) pseudomorphic replacement of scheelite by poorly crystalline WO3.xFe2O3.nH2O (iron-containing meymacite) at pH ~4–1; (3) crystallization of tungstite, WO3. H2O, and hydrotungstite, WO3.2H2O, at the expense of meymacite at pH <1. The second group of processes proceeds in the overall trend of increasing pH of supergene solutions and includes: (1) partial dissolution of meymacite upon increasing the pH and the K, Na and W content in the solution (pH ~1–3); (2) occasionally, precipitation of amorphous gels WO3.xFe2O3.nH2O – chemical counterparts of meymacite (pH ~1–3); (3) crystallization of hydrokenoelsmoreite, (W,Fe)2(O,OH)6.H2O – the second chemical counterpart of meymacite with the pyrochlore structure type, as result of the interaction of meymacite and solution enriched with K, Na, Ca and, most probably W, at pH ~3–4; (4) formation of mineral bearers of W: goethite, α-FeOOH, hematite, α-Fe2O3, undefined amorphous SiO2.xAl2O3.yFe2O3.nH2O gels, and, rarely, of CaWO4, and stolzite, PbWO4, at pH>4. It is shown that the increase of pH from <1 to 4 causes the successive change of the structure types: ReO3 (ReO3-type layers in the structures of tungstite and hydrotungstite), hexagonal tungsten bronze (HTB) (HTB-type layers in the structure of WO3.1/3H2O, meymacite), and pyrochlore (structure of hydrokenoelsmoreite). Keywords: scheelite alteration, oxidation zone, secondary tungsten minerals, structure types. INTRODUCTION structure [2] and poorly crystalline WO3.1/3H2O [6]. The obtained so far data show that the structures In 1981, Th. G. Sahama in his review [1] devot- of the secondary W minerals of the type (W,Fe) ed to the secondary W minerals wrote that “The sec- (O,OH)3.nH2O and their artificial counterparts have ondary tungsten minerals form a group of species similar structure elements such as layers of hex- with no crystallographic interrelationship”. For the agonal tungsten bronze or/and ReO3 perovskite [5]. decades since then, new data on the secondary W Such minerals as pittongite and phyllotungstite [3, minerals were collected and a number of new sec- 4] are characterized with combined pyrochlore and ondary W minerals with simplified formula (W,Fe) tungsten bronze types of structure. (O,OH)3.nH2O (hydrokenoelsmoreite, pittongite) Thus, the crystallographic interrelationships be- were discovered [2–4]. Several new phases of tween the secondary W minerals do exist. However, WO3.nH2O (1/3, 1/2) with structures of hexagonal it is difficult to relate these minerals and their struc- tungsten bronze and pyrochlore were synthesized ture to certain physicochemical conditions. The using the soft chemistry (chemie douce) approach synthesis conditions can hardly be used directly to [5]. These new phases have their natural analogues interpret the natural conditions as they depend on as hydrokenoelsmoreite with а pyrochlore type of numerous geological factors. The processes of weathering occurring in W deposits cause a significant change in the mineral * To whom all correspondence should be sent: composition and properties of W ores, which makes E-mail: [email protected] them unsuitable for flotation and gravity beneficia- 270 © 2018 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria M. P. Tarassov, E. D. Tarassova: Structural and chemical evolution of mineral forms of tungsten in the oxidation zone... tion. The behavior of W in the oxidation zone is very of pyrite and the generation of natural sulfuric acid. complicated due to complex aquatic chemistry of Goethite, α-FeOOH, and jarosite, KFe3(SO4)2(OH)6, the element and its ability to polymerize. Recently, are two most dominated minerals in the oxidation W has become the subject of scientific focus due zone of the deposit indicating very high variation to the possible toxicity of W in drinking water [7]. of pH of the supergene solution – from neutral to Weathering of W deposits is considered as one of strongly acid, and Eh being very close to the val- the possible sources of W in ground waters [8]. ues typical for waters being in direct contact with The processes of weathering affect signifi- atmospheric oxygen [10]. All altered ores in the cant part of the Grantcharitza deposit (Western oxidation zone are clearly divided into two groups: Rhodopes), the largest tungsten deposit in Bulgaria. limonitized ones with dominated ferric iron oxides/ In the present paper, the authors discuss the prin- oxyhydroxides and non-limonitized ones, contain- cipal features of the development of the oxidation ing jarosite and a variety of secondary tungsten zone in the Grantcharitza tungsten deposit and the minerals. structural and chemical evolution of different sec- ondary mineral forms of W by connecting them with certain physicochemical conditions. MATERIAL AND METHODS Representative samples from а personal collec- BREAF INFORMATION ABOUT THE tion of the authors taken from the oxidation zone GRANCHARITSA DEPOSIT of the Grantcharitza deposit (Center section) are used for the present study. Scanning electron mi- The Grantcharitza tungsten deposit is situated in croscopy and electron probe microanalysis (Philips the Western Rhodopes Mountains, 18 km southwest SEM515 – WEDAX3A and ZEISS EVO LS25 of the town of Velingrad (Plovdiv region, Bulgaria). – EDAX Trident) at 15–20 kV acceleration volt- The deposit is localized in porphyritic biotite gran- age, and micro-Raman spectroscopy (MicroDil ites and amphibole-biotite granodiorites of the 28 (Dilor Co.) with an Olympus 100x microscope so-called “unit 1” of the composite Rila-Western objective, 488-nm line of an Ar+ laser, laser power Rhodopes Batholith [9]. The ore mineralizations below 2 mW at the sample surface) were the main occur in pegmatoid quartz-feldspar veins charac- methods for the sample characterization. For a part terized by almost sub latitudinal strike – dipping of the samples, transmission electron microscopy to NW ~350° with slope of ~30° and in the wall- (Philips TEM420) at 120 kV and Powder X-Ray rock as vein-disseminated ores (“mineralized grani- diffraction analysis (DRON-UM1, CoKα and CuKα toids”). The mineral composition of the vein ores is radiations) were also applied. For better understand- characterized by strong domination of quartz, SiO2, ing the supergene processes in the oxidation zone and potassium feldspar, KAlSi3O8 (microcline) of the deposit, the authors have constructed series among the gangue minerals and of pyrite, FeS2, and of Eh–pH diagrams for the systems W–Ca–Fe–S– scheelite, CaWO4, among the ore minerals. The re- K–O–H and W–Fe–O–H at 298 K and 1 atm. us- gion of the deposit is characterized by broken ter- ing the thermodynamic data from [11–13] and the rain which causes fragmentariness of the weather- activities of chemical components (ΣFe – 10–4, ing crust. The oxidation zone of the deposit is devel- Ca2+ – 10–4, ΣS – 10–2, K+ – 10–3) in aqueous solu- oped unevenly. Most intensive supergene processes tions typical for the oxidation zones of ore deposits are observed in the southern upper area of the most [10, 14]. The thermodynamic activity of W for the economically important Grantcharitza-Center sec- system W–Fe–O–H was chosen equal to 10–5 – the tion of the deposit. There, a significant part of the value corresponding well to the CaWO4 solubility relief is presented by a gentle slope (~30°) of the in water according to [15]. For simplification, only 2– Grantcharitza River valley. This slope is subparallel the monomeric tungstate ion [WO4 ] was taken into to the ore zone and thus ensures nearly even access account. to the ore zone of the weathering agents – water, at- mospheric oxygen, microorganisms, etc. The textural and structural features of the primary ores and their RESULTS AND DISCUSSIONS mineral composition (presence of quartz and potas- sium feldspar which screen the ore minerals from the The following W minerals and W-bearing min- weathering agents) contribute to the inhomogeneous erals are established in the oxidized ores: (i) hypo- development of the supergene processes which are gene minerals – scheelite CaWO4 (intact and relic controlled by the fracture zones in the ores. – most common); (ii) supergene minerals: iron-con- The most important supergene process that ul- taining meymacite WO3.xFe2O3.nH2O (important), timately modifies the primary ores is the oxidation tungstate WO3.H2O, hydrotungstite WO3.2H2O, 271 M. P. Tarassov, E. D. Tarassova: Structural and chemical evolution of mineral forms of tungsten in the oxidation zone... 10– 6– iron-containing hydrokenoelsmoreite (ferritung- [H2W12O42] , α-metatungstate [H2W12O40] and stite) (W,Fe)2(O,OH)6.H2O, stolzite PbWO4 (rare), other isopolytungstate ions. The role of monomer- 2– colloform supergene scheelite CaWO4 (rare), amor- ic forms of tungsten WO4 is significant at pH>6. phous WO3.xFe2O3.nH2O gels; (iii) mineral bearers These data show that, at least at the beginning stages of tungsten: goethite α-FeOOH (most widespread), of scheelite alteration, at pH<6 to about 4 the only hematite α-Fe2O3, amorphous SiO2.xAl2O3.yFe2O3. process of changing the mineral is its dissolution nH2O gels. which proceeds with the formation of polytungstate Scheelite, CaWO4, (endogenic) is the most im- ions.