Sandwich” Wulfenite from the Ojuela Mine, Mapimí, Mapimí Municipality Durango, Mexico: Evidence of Preferred Secondary Nucleation on Selected Wulfenite Faces
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Philippine Journal of Science 150 (2): 527-533, April 2021 ISSN 0031 - 7683 Date Received: 08 Sep 2020 Characterization of “Sandwich” Wulfenite from the Ojuela Mine, Mapimí, Mapimí Municipality Durango, Mexico: Evidence of Preferred Secondary Nucleation on Selected Wulfenite Faces J. Theo Kloprogge1,2* and Jessebel P. Valera-Gadot3 1Department of Chemistry, College of Arts and Sciences University of the Philippines Visayas, Miag-ao, Iloilo 5023 Philippines 2School of Earth and Environmental Sciences, The University of Queensland Brisbane 4072 Queensland, Australia 3Material Science and Nanotechnology Laboratory Regional Research Center – Office of the Chancellor University of the Philippines Visayas, Miag-ao, Iloilo 5023 Philippines This paper describes the formation of “sandwich” wulfenite. Banded wulfenite from the Ojuela Mine, Mapimí, Durango, Mexico, have been found since 2017, but an explanation for the band formation has not been provided. X-ray diffraction (XRD) showed the wulfenite to have a tetragonal unit cell of a = 5.4374(1), c = 12.1123(7) Å. The Raman spectrum was dominated by –1 –1 ν1 (Ag) around 870 cm , while the weak shoulder at 859 cm represents the strain activated ν1 –1 –1 (Bu) infrared (IR) band. Two ν2 modes were observed at 318 cm (Ag) and 351 cm (Bg). For the –1 –1 ν3(Eg), the band at 768 cm was assigned to the Bg symmetric mode and the 745 cm band to the –1 Eg vibration. The IR spectrum showed two strong bands at 835 and 779 cm corresponding to –1 v3 modes and a very weak one at 496 cm . SEM-EDX (scanning electron microscopy – energy dispersive X-ray analysis) showed that the band does not extend into the main crystal but is limited to the surface as secondary growth. The main thick tabular crystal was dominated by {001}, {110}, and {111}. The {001} surfaces of the wulfenite crystals showed evidence of later dissolution, which may have been part of the source material for this secondary crystallization. The chemical composition of the main wulfenite crystal proved only minor substitution of W for Mo. Contrastingly, a much higher number of substitutions by V, As, and W for Mo – as well as Ca, Fe, Zn, Cu, and Al for Pb – was observed in the secondary band, indicating a change in the fluid composition from which the band crystallized compared to the original wulfenite. Changes in lattice parameters of the secondary wulfenite crystals and the energy involved in nucleation on the surface of the original wulfenite as well as the different bonds exposed on different faces favored the formation of the band on only {110}. Keywords: FT-IR spectroscopy, molybdate, Raman spectroscopy, secondary crystallization, SEM- EDX, wulfenite *Corresponding Author: [email protected] 527 Philippine Journal of Science Kloprogge and Valera-Gadot: Preferred Secondary Vol. 150 No. 2, April 2021 Nucleation on Wulfenite Faces INTRODUCTION more during this process resulted in many, often very large open spaces in which these secondary minerals Molybdenum is a very scarce element with a crustal could freely crystallize. During the 334 years between abundance of only about 1 ppm. It is mainly used 1598–1932, the district produced a total of around 6 in steel where it provides hardness and resistance to million tons of high-grade, silver-rich oxide ores and abrasion, corrosion, and high temperatures. Further, it perhaps as much as a million tons of sulfides (Megaw is used in pigments, catalysts, and lubricants (Blossom et al. 1988). Overall production grades were 15.0% Pb, 1985; Kesler 1994). Wulfenite, Pb(MO4), was originally 475 g/T (= 15 troy ounces) Ag, and 3.5 g/T (= 0.1 troy named "plumbum spatosum flavo-rubrum, ex Annaberg, ounce) Au. Copper grading up to 3% was produced in Austria" in 1772 by Ignaz von Born (von Born 1772). In limited areas (Campos et al. 1988). Zinc was a prominent 1781, Joseph Franz Edler von Jacquin called the mineral component of the ores, probably averaging 12–15%, but "Kärntherischer bleispath." In 1845 Wilhelm Karl von it was never recovered. Wulfenite is widely distributed in Haidinger renamed it wulfenite in honor of Franz Xavier the oxidized lead ores, associated with minerals such as von Wulfen (05 Nov 1728, Belgrade, Serbia – 16 Mar mimetite, duftite, bindheimite, cerussite, hemimorphite, 1805, Klagenfurt, Austria), botanist, mineralogist, aurichalcite, adamite, plattnerite, and calcite (Hoffman alpinist, and a member of the Order of the Society of 1968). The color range is very wide, but the most Jesus (Jesuit) (von Haidinger 1845). Wulfen authored a common hues are yellow, orange, and brown. monograph on the lead ores of Bleiberg, Austria (von Wulfen 1785). Raman and IR spectroscopies have been proven to be especially useful for the study of minerals. Raman Wulfenite is a secondary mineral typically found as spectroscopy has been shown to be most useful for the thin tabular crystals with a bright orange-red, yellow- study of diagenetically and structurally related minerals orange, yellow, or yellowish-grey color (though rarely such as minerals containing molybdate and tungstate as colorless, grey, brown, olive-green, and even black) groups. In our earlier work the Raman spectroscopy of in the oxidized zones of hydrothermal lead deposits. wulfenite (PbMO4), tungsteinian wulfenite, scheelite Occasionally, crystals form dipyramids or thick tabular (CaWO ), and wolframite [ferberite-hübnerite series, crystals. It crystallizes in the 4/m point group. In unique 4 (Fe, Mn)WO4] have been described in details (Crane et situations, it may precipitate from volcanic fumarole al. 2002; Kloprogge and Frost 1999). gas, in temperatures from 400–550 °C (Africano et al. 2002). The Ojuela Mine, Mapimí, Mapimí Municipality, In recent years, finds of so-called “sandwich” wulfenite – Durango, Mexico, is well known among mineralogists where butterscotch colored crystals show a darker brown and mineral collectors around the world and is probably band around the sides of the tabular crystals resembling a Mexico’s greatest mineral locality. It was originally sandwich – have been described from the famous Ojuela discovered by the Spanish conquistadores in 1598 Mine, Mapimí, Mapimí Municipality, Durango, Mexico. (although there are some indications of rudimentary The main find of this material was back in 2017 but more mining by early native American peoples); the deposit recently smaller pockets containing similar material have was actively mined for 350 years but did not come to been found. Some mineral dealers have described the notice of mineralogists until W.F. Foshag’s visit in 1927 banded structure as a flat tabular crystal with later epitaxial (Rice 1908a, b; Waszkis 1993) The district is a typical growth on the (001) and (001) faces. In this paper, a Ag-Pb-Zn (Cu-Au) carbonate replacement deposit detailed characterization of these wulfenite crystals using (Megaw et al. 1988; Megaw 1999; Prescott 1926). XRD, IR, and Raman spectroscopy, and SEM combined Mineralization in these deposits occurs predominantly with EDX analyses will be provided and an attempt will by replacement, a process in which hot, acidic, and saline be made to explain the observed banded structure. ore fluids dissolve limestone or dolomite around the fractures along which they migrate, and the neutralization of the fluids causes almost immediate precipitation of sulfide or silicate minerals in the resulting micro-voids. MATERIALS AND METHODS The major ore minerals are pyrite, pyrrhotite, galena, sphalerite, chalcopyrite, arsenopyrite, and silver sulfides Sample and sulfosalts. Throughout northern Mexico, relatively The wulfenite sample used in this study is from recent (younger than 20 Ma) regional block faulting the first author’s collection under catalog has uplifted many replacement deposits to levels where no. SU70. The wulfenite comes from the Ojuela descending meteoric water could attack the primary Mine, Mapimí Municipality, Durango, Mexico, and sulfide minerals and oxidize them to a wide range of consists of tabular butterscotch colored crystals with secondary minerals. A volume reduction up to 20% or darker brown bands around the crystals (Figure 1). 528 Philippine Journal of Science Kloprogge and Valera-Gadot: Preferred Secondary Vol. 150 No. 2, April 2021 Nucleation on Wulfenite Faces function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Gauss-Lorentz cross- product function with the minimum number of component bands used for the fitting process. The Gauss-Lorentz ratio was maintained at values greater than 0.7, and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995. SEM was performed on a Hitachi SU3500 equipped with a Bruker QUANTAX 200 EDX Spectrometer, which includes a 129-eV XFlash 6l30 detector system. The uncoated wulfenite sample was mounted on a 26 mm sample holder with a carbon tape to hold the sample under vacuum pressure. The surface morphology of the sample was analyzed with an accelerating voltage of 15–20 kV, a beam current of 100–180 A, working distances of 10.2–19.4 mm, and at low magnifications of 23–900x. The elemental composition of푢 the wulfenite was carried out by point, line, or mapping EDX analyses. In addition, the accelerating voltage was lowered to 1.5 kV at high Figure 1. Wulfenite SU70 showing the blocky, tabular crystals with magnifications to minimize charging on the sample. the darker brown bands, field of view about 1 cm. Sample Analyses RESULTS AND DISCUSSION The nature of the crystals was determined by XRD The XRD pattern of one of the wulfenite sample is prior to the spectroscopic analyses. XRD patterns of the shown in Figure 2. The pattern is consistent with that powdered samples were recorded on a Philips wide-angle of pure wulfenite. The unit cell was calculated based on PW1050/25 vertical goniometer applying Cu K radiation. α the tetragonal 4/m crystal system with a = 5.4374(1) and The samples were measured at 50% RH in step-scan mode c = 12.1123(7) Å.