Vesuvianite from the Metamorphic Aureole of the Panticosa Pluton (Huesca, Spain) ELISENDA RODRÍGUEZ PÉREZ (1*), JOSÉ F
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macla nº 21. 2016 revista de la sociedad española de mineralogía 76 Vesuvianite from the metamorphic aureole of the Panticosa Pluton (Huesca, Spain) ELISENDA RODRÍGUEZ PÉREZ (1*), JOSÉ F. BARRENECHEA (2)(4), CÉSAR CASQUET (3)(4), ELENA CRESPO(2) (1) Clariant Ibérica Producción, S.A. Camino de la Magdalena s/n, 45210, Yuncos, Toledo (Spain) (2) Departamento de Cristalografía y Mineralogía. Facultad de Ciencias Geológicas. José Antonio Novais, 12. Universidad Complutense de Madrid. 28040, Madrid (Spain) (3) Departamento de Petrología y Geoquímica. Facultad de Ciencias Geológicas. José Antonio Novais, 12. Universidad Complutense de Madrid. 28040, Madrid (Spain) (4) Instituto de Geociencias IGEO (CSIC-UCM). coordination and are mainly occupied by INTRODUCTION Ca2+, although possible substitutions by Na+, La3+ (REE), Pb2+, Bi3+ y Th4+ have Devonian marbles within the internal been described (Groat et al., 1992). For aureole of the Panticosa Pluton contain the Y positions, the most relevant cation good examples of vesuvianite that were is Al3+, with some important amounts of overlooked. The pluton emplacement others elements as Mg2+, Fe2+, Fe3+, provoked recrystallization of the regional sometimes Ti4+, and less frequently low-grade organic matter-bearing other metallic elements as Cr2+, Mn2+, limestones and the formation of Cu2+ or Zn2+. skarnoids at the expenses of interbedded siliclastic layers. The latter fig 1..Panticosa pluton geologic map, where the red The refined structure of the vesuvianite contain large crystal of vesuvianite. square indicates the study area. Modified from without B- shows the presence of 78 Other minerals in addition to vesuvianite Santana (2002). anions (a.p.f.u), where 68 are are clinopyroxene, wollastonite, and unmistakably O2-, based on the joins graphite, among others. Graphite is VESUVIANITE valences (Groat et al., 1992). The W abundant in both marbles and corresponds to O2-, (OH)- or F-. skarnoids. Vesuvianite or idocrase is a rare rock- Accordingly, the ionic charge range from forming or accessory silicate mineral 146- to 156-, depending on the number The subject of this work is the mainly found in skarns, rodingites and of OH-, from ten to cero, respectively. mineralogical and chemical description altered alkali syenites. The vesuvianite of vesuvianite, i.e., the more significant crystalline structure was determined by It is relevant to point out that mineral of the skarnoid layers. Relevant Warren and Modell (1931), who found vesuvianite may contain a variable variations in color, optical and textural an arrangement similar to the structure amount of B in the structure, thus features and mineral assemblages are of grossular structure, but including producing a significant variation in the confronted with EPMA compositional additional atoms on the fourfold axes. general formula (2): data. This resulted in a lower symmetry, which corresponds to space group P4/nnc. X19Y13T (0-5) Z18O68 W10 (2) GEOLOGICAL SETTING However, there still are some uncertainties related to the actual Where the boron positions correspond to The marble spot referred to here is symmetry and complexity of the the T, from cero to five. located within the metamorphic aureole crystalline structure. Several crystal- at the southern contact of the Panticosa chemical formulae have been proposed. As mentioned above, vesuvianite is a Pluton on the road from the Panticosa The huge range of possible cationic mineral with a wide range of Spa to the homonymous town (Fig. 1). substitutions generated the major compositional variations, thus allowing The latter pluton is part of a complex difficulty to adequate a general formula the existence of crystals with differences formed by several granitic plutons for this mineral. in physical properties like color and between Cauterets (France) and optical behavior based on their Panticosa (Spain) in the Variscan In order to generate a structural chemistry, even if the mineral structure basement of the Pyrenees in the Axial formula, the cations are assigned to and symmetry are preserved. Zone (Santana, 2002). The more recent three different coordination groups radiometric age of the complex (using according to their charge and atomic Materials and Methods the U-Pb method), yielded ca. 301 My ratio, following the schematic formula (Ternet et al., 2004). Host rocks to the (1): X-ray diffraction (XRD) and optical pluton belong to the regional low-grade microscopy was used to characterize the Larrue Black Limestones and to the X19Y13Z18O70W8 (1) mineral assemblage in each sample. detrital series Facies Sia. Both series are The textural relationships were middle Devonian (Bresson, 1903). The Z sites are in tetrahedral established through a detailed coordination and are occupied by Si4+, petrographic study. The analytical with some minor substitutions by Al4+. methods used for the chemical The X locations show [8]- or [9]- fold palabras clave: Vesubiana, Idocrasa, Panticosa, Química mineral. key words: Vesuvianite, Idocrase, Panticosa, Mineral chemistry. Jornada SEM * corresponding author: [email protected] macla nº 21. 2016 revista de la sociedad española de mineralogía 77 charT acterization were scanning electron prenhite, calcite, wollastonite, graphite emplacement but flattening probably microscopy (SEM) and electron and minor amounts of quartz and pyrite reactivated during intrusion provoking microprobe analyses (EMPA). Due to the (Fig. 2). some deformation of the vesuvianite limitations of these techniques, it was crystals. These competent beds can be not possible to measure the amount of further correlated with fine grained structural water and light ions like B. All sandstones interbedded with marbles the iron measured was assumed as that outcrop on the same road outside Fe2+, the sulfur as SO3, and the H2O the metamorphic aureole. content has been calculated according to the balanced charges for 78 a.p.f.u. Mineral Chemistry anions. A comparison between the two different The first proposed formula was types of vesuvianite found inside the calculated based on 78 oxygen atoms contact aureole has been performed. (Coda et al., 1970; Rucklidge et al., SEM and EPMA analyses have allowed 1975). This is a preliminary useful fig 2. Skarnoid sample with paragenesis MP1, with determine the chemical differences in method, but since the vesuvianite graphite patches around brown idiomorphic crystals with distinct colors. structural formula can contain F-, Cl-, and vesuvianite crystals. OH- as additional anions substituting for The brown vesuvianite crystals show O2-, an alternative properly formulation The second paragenesis (MP2) is formed higher Fe contents (Fe-vesuvianite is based on the reduction to 50 cations, mainly by yellow to green vesuvianite henceforth), however the green to yellow excluding the B (Hoisch, 1985). In the and calcite (Fig. 3), together with vesuvianite crystals contain lesser Fe case of Panticosa, both methods have fragmented clinopiroxene and prenhite and higher amounts of Mg (Mg- been proved and the resulting formulae crystals cemented by recrystallized vesuvianite) Al, F and Cl. A relevant point show an excess on the ionic charges calcite. It is remarkable the absence of is that the same chemical variation valence and of Ca2+ in the X cation graphite and the presence of pyrite trend is shown by clinopyroxene from positions, which cannot be fit in another crystals in this assemblage. each paragenesis. Cpx from PM1 (Fe- place due to its large size. For this vesuvianite) and PM2 (Mg-vesuvianite) reason, the used method has been a show higher iron and magnesium normalization to ΣX (Ca2+) = 19 cation contents, respectively. p.f.u. (Groat et al., 1992). Selected analyses from both types of MINERAL PARAGENESIS vesuvianite are shown in Table 1. Briefly, the Fe-vesuvianite has a large variation Vesuvianite dealt with here is found in range among the different measured two mineral paragenesis into the crystals, being most remarkable in the restricted area occupied by the aureole following oxides: FeO (2.13-5.53 wt. %), of the Panticosa pluton. In both cases fig 3. Sample with Paragenesis MP2. Blackish MgO (1.15-2.57 wt. %), TiO2 (0.14-2.21 the crystals morphology, size and setting marble (without graphite) in the upper half, and wt. %), SO3 (0.02-0.17 wt. %) y Cl (0.15- are similar, i.e., isolated idiomorphic calcite veins and yellow vesuvianite prismatic 0.28 wt. %). In contrast, Mg-vesuvianite prisms 1 to 4 cm long with a perceptible crystals in the lower part. has a lower range of chemical variation zoning. with a lesser FeO content (0.41-0.67 wt. Vesuvianite skarnoids formed at the %) and higher MgO (2.59-2.89 wt. %), expense of strongly folded competent The most common paragenesis (MP1) SO3 (0.37-0.67 wt. %) and Cl (0.25-0.49 beds within the Devonian marbles. consists of brown vesuvianite, diopside, wt. %) values, variations in TiO2 (0.07- Folding was in part older than the pluton 2.12 wt. %) and F (1.31-1.89 wt. %) also Fe-Vesuvianite* (MP1) Mg-Vesuvianite** (MP2) remarkable. SiO2 36.2 36.17 Al2O3 16.59 18.33 FeO 2.44 0.58 MnO 0 0.11 MgO 2.57 2.78 CaO 36.78 36.83 Na2O 0.04 0.02 K2O 0.01 0.01 TiO2 1.72 1.07 NiO 0.02 0 Cr2O3 0.03 0.01 SO3 0.08 0.6 F 1.62 1.56 Cl 0.18 0.43 -O≈ F,Cl 0.72 0.76 fig 4.. Triangular plot with both vesuvianite Total 97.55 97.73 populations, for the Fe-Mg-Ti elements (a.p.f.u). *Ca19.00(Al8.88Mg1.85Fe0.98Ti0.62)Al0.55Si17.45O68(OH)7.39F2.47Cl0.14 Figure 4 shows a triangular plot of the **Ca19.00(Al9.92Mg2.00Fe0.23Ti0.39)Al0.58Si17.42O68(OH)7.28F2.37Cl0.35 Table 1. Representative analyses and mineral formulae calculated for the both types of vesuvianite, based on Fe, Ti and Mg contents in both crystal 19X cations (without S). populations, with distinguishable macla nº 21.