scanning electron microscopy (sem) and energy dispersive x...

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray analysis (EDX) of Daughter Minerals in Fluid Inclusions in Layered Silicate Materials

A. Ruiz-Conde1, E. Garzón2 and P.J. Sánchez-Soto3

1Instituto de Ciencia de Materiales de Sevilla (ICMS), centro mixto Consejo Superior de Investigaciones Científicas (CSIC)-Universidad de Sevilla, c/Américo Vespucio 49, 41092-Sevilla, Spain,

e-mail: [email protected]; [email protected] 2Departamento de Ingeniería Rural, Universidad de Almería, La Cañada de San Urbano, 0412-Almería, Spain

The analysis of fluid inclusions is important to understand the geological processes that have taken place in the rock and minerals host. This kind of study is an interesting topic in which the microscopy methods are involved. Fluid inclusions provide the most reliable information on the characteristic of ore-forming fluids, including the temperature and pressure of the fluid, as well as compositional information such as salinity, metal concentration and gas fugacities. Fluid inclusions in hydrothermal minerals and synthesized inorganic materials often contain solid phases considered daughter minerals, which precipitated during cooling from the high temperatures of fluid entrapment.

Fluid inclusions in minerals are typically smaller than 100 μm in diameter, and are commonly observed by optical microscope if trapped in transparent minerals. Daughter minerals in opened fluid inclusions can be identified using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX). However, daughter minerals are smaller (ca. 1 to 10 µm across) and there are often many minerals within the cavity of the opened inclusion. The analyzed surface of an individual crystal is usually not perpendicular to the incident beam. In addition, instrument components and other effects of elements in surrounding minerals leads to qualitative or semiquantitative compositional analysis.

Studies of fluid inclusions and daughter minerals in layered silicate materials have been very scarce and little information by SEM-EDX can be found in the literature. In this chapter, daughter minerals in fluid inclusions identified in samples of layered silicates phlogopite and vermiculite (Santa Olalla deposit, Huelva, SW Spain), have been studied by SEM-EDX. The main observations by optical microscopy and SEM of daughter minerals in fluid inclusions are described for two layered silicates phlogopite and vermiculite using large transparent flakes avoiding the preparation of thin polished sections. EDX analysis has been used to achieve information on chemical composition of the observed crystallization and morphologies and, hence, to identify the possible mineral phases.

Cubic crystals in phlogopite, as observed by SEM in solid inclusions of variable sizes (50-100 μm), have been analyzed by EDX being identified as halite (NaCl). Other smaller crystals, with rounded morphologies and sizes 3-15 μm, even as dendritic form, as observed by SEM, have been identified by EDX as sylvite (KCl). Moreover, it has been also observed and analyzed NaCl and KCl mixed in some phlogopite flakes. The effect of the incident electronic beam in thin phlogopite flakes is not negligible and rapid heating effects are produced which influenced the crystallization of the fluid inclusion.

Fluid inclusions appear in vermiculite flakes after examination by SEM with daughter minerals showing cubic, prismatic and acicular or fibrous forms. They appear sometimes in the interior of opened and broken vermiculite flakes, in like-triple points and/or surface expanded vermiculite flakes. Cubic crystals have been identified by EDX analysis as halite (NaCl), and prismatic crystals as magnesium dichloride. Acicular and fibrous crystals as observed by SEM in vermiculite flakes have been also analyzed by EDX, being identified as calcium sulphate (gypsum, anhydrite). It appears sometimes mixed with Ca and/or Mg chlorides according to EDX results.

The present study shows the feasibility of SEM-EDX investigation applied to the study of daughter minerals in fluid inclusions, as identified in layered silicate materials, such as phlogopite and vermiculite using large transparent flakes.

Keywords fluid inclusions, daughter minerals, layered silicates, crystallizations, phlogopite, vermiculite, halite, sylvite

1. Introduction

Fluid inclusions in minerals represent hydrothermal solution trapped during precipitation of the crystal [1-8]. At room temperature, they contain both liquid and vapour. Fluid inclusions in hydrothermal minerals and synthesized inorganic materials often contains solid phases, considered daughter minerals, which precipitated during cooling from the high temperatures of fluid entrapment. The analysis of fluid inclusions and their daughter minerals is important to understand the geological processes that have taken place in the rock and minerals host [8]. This kind of study is an interesting topic in which the microscopy methods are involved [1-7]. Fluid inclusions provide the most reliable information on the characteristic of ore-forming fluids, including the temperature and pressure of the fluid, as well as compositional information such as salinity, metal concentration and gas fugacities [3-7]. These data make it possible to clarify the mechanism of ore formation in terms of metal transport, fluid cooling and mineral precipitation [7,8]. Fluid inclusions in minerals are typically smaller than 100 μm in diameter, and are commonly observed by optical microscope if trapped in transparent minerals [3,6]. Daughter minerals in opened fluid inclusions can be identified using

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Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis [2,4,5]. However, daughter minerals are smaller (ca. 1 to 10 µm across) and there are often many minerals within the cavity of the opened inclusion. It should be noted that the analyzed surface of an individual crystal is usually not perpendicular to the incident beam. In addition, instrument components and other effects of elements in surrounding minerals leads to qualitative or semiquantitative compositional analysis better than quantitative [4-6,8]. Identification of daughter minerals in fluid inclusions usually depends upon externally observed properties, such as solubility, crystal morphology, and birefringence [3,6]. Other techniques have been employed. These include the extraction and X-ray diffraction analysis of individual daughter minerals [9], Raman spectrometric analysis [10], synchrotron X-ray fluorescence [8], Transmission Electron Microscopy (TEM) [11] and the use of SEM and EDX [2-6]. Usually, polished thin sections (and/or the cores cut and polished on both sides) to prepare sections (0.25 thick) must be used. The combined use of SEM-EDX techniques is attractive because information on the morphology of the crystals and the elements present can be obtained in a relatively short time. A variation on daughter mineral analysis by SEM involves converting the entire inclusion to “daughter minerals” by freezing with liquid nitrogen [6]. Although analysis can be done on extracted material [3,6], a simpler approach which is used in most studies is to break samples of host mineral and study the exposed daughter minerals in situ on fracture surfaces. Studies of fluid inclusions and daughter minerals in layered silicate materials have been very scarce and little information by SEM-EDX can be found in the literature. According to Roedder [3,6], this kind of studies in layered silicates, such as micas, is difficult. This author [3] described fluid inclusions in biotites and phlogopites. Anthony et al. [4] studied daughter minerals in fluid inclusions as found in hydrothermal quartz. Fragments of quartz known to contain fluid inclusions having daughter minerals were broken (using a small anvil) and mounted according to standard SEM techniques. Concerning silicates, these authors tentatively identified potassium feldspar, muscovite (potassium mica), a trioctahedral iron mica belonging to the siderophyllite-annite series and daughter minerals identified as dioctahedral muscovite according to their EDX analysis. They used the raw EDX elemental peak intensities of the daughter minerals compared to those obtained for reference samples of the minerals. Other authors [12] described fluid inclusions in phlogopites in positions parallel to [00l], besides cubic crystals of halite or anhydrite. For instance, Loucks [13] studied the content of water (hydration) in micas. He found submicroscopic fluid inclusions filled with water. Consequently, it can not be distinguish between interlaminar water of the layered silicate and water coming from fluid inclusions using techniques such as Infra-Red (IR) or Thermal Analysis (TA). In this chapter, daughter minerals in fluid inclusions identified in samples of two layered silicates phlogopite and vermiculite have been studied by SEM-EDX. Phlogopite and vermiculite (samples from Santa Olalla deposit, Huelva, SW Spain) have been selected taking into account previous investigations [14-19]. The main observations by optical microscopy and SEM of daughter minerals in fluid inclusions are here described by the first time for both phlogopite and vermiculite using large and transparent thin flakes. EDX analysis has been used to achieve information on chemical composition of the observed crystallization and morphologies and, hence, to identify the possible mineral phases.

2. Materials, methodology and microscopy techniques

Two layered silicate materials have been examined and studied: phlogopite and vermiculite. The selected phlogopite and vermiculite samples are presented as characteristic large transparent flakes (Fig. 1). They have been obtained from a deposit at Santa Olalla (Huelva, SW Spain). The flake sizes are typically 150x50 mm (or even larger) with thickness 10-20 mm. The original raw samples were carefully washed with deionized water, and thus to eliminate external powdered earth, iron oxides and impurities present at the surface, and air-dried. Thin large and transparent flakes of the internal part were carefully separated by the same operator using scalpels from thick larger samples. Previous studies on these materials have been considered, in particular the identification of both layered silicates by X-ray Diffraction (XRD) methods using large flakes and in ground powders [16-19]. Structural identification of phlogopite and vermiculite by characteristic X-ray diffraction patterns demonstrated the purity of both samples as silicon-aluminium and iron-magnesium silicates containing potassium, in the case of phlogopite, and additional magnesium in the case of vermiculite. Chemical analysis by Atomic Absorption Spectrometry allowed to determine the composition of both samples (major elements) including possible element impurities and cation exchange elements and, hence, the structural phormulae (one half of the layer) as follows: Phlogopite: (Si3.15Al0.85)(Mg2.08Al0.45Fe3+

0.014Fe2+0.18Ti0.025Mn0.009) O10 (OH)2 (Na0.03K0.70Ca0-07)

Vermiculite: (Si3.14Al0.86)(Mg1.77Al0.63Fe3+

0.07Fe2+0.18Ti0.015Mn0.006)O10(OH)2(Mg0.38K0.002Ca0.015Na0.004)

The thermal weight loss (until consistent weight is reached) of 1.0000 g of phlogopite and vermiculite, as ground powdered samples using and agate mortar, after heating at 1000 ºC during 1 hour using a Pt crucible in an electric



furnace (HD 230 PAD equipment), showed values of 4.96 wt % and 20.63 wt %, respectively by this TA method. They were in good agreement with the weight loss of theoretical phormulae of phlogopite and vermiculite. Previous observations using optical and petrographic microscopies, because the large thin flakes of phlogopite and vermiculite are optical transparent (Figure 1), allowed to investigate the number and variable distribution of fluid inclusions. Visible light microscopy (optical petrographic microscopy) was performed using a polarizing microscope Nikon, model Optiphot-2 Pol and magnifications 10x, 20x, 40x and 100x. Selected micrographs were obtained from optical petrographic microscopy observations using a Nikon digital photographic camera, model FX-35DX. SEM examination of the samples was conducted using a JEOL scanning electron microscope, model JSM-5400 and chemical analysis under the microscope using an Oxford Link energy dispersive X-ray detector (EDX), model ISIS with Si(Li) detector and thin Be window. The filament for the electronic beam source was W at 20 kV as maximum. Phogopite and vermiculite samples as large thin flakes, previously examined using optical microscopy, were deposited using graphite adhesive and coated with Au using a Sputter Coater equipment, model Emitech 550. Gold coating provides better resolution and is more frequently used in SEM studies as compared with carbon coating although some interferences between elemental peaks can be avoided using the second one [2,4-6].

a) b) c)

Fig. 1 Optical micrographs obtained by petrographic microscopy of (a) phlogopite, (b) vermiculite and (c) other representative sample of vermiculite using large (ca. 40 x 20 mm) transparent flakes (0.2 mm thin). Magnification 20 X.

3. Results and discussion

3.1. Phlogopite

The main observations by optical microscopy and mainly SEM of daughter minerals in fluid inclusions are described for phlogopite and vermiculite. EDX analysis has been used to achieve information on chemical composition of the observed SEM crystallizations and morphologies and, hence, to identify the possible mineral phase. The problem was more complex and difficult when several crystallized mineral phases are present in the samples. However, in contrast to previous studies on fluid inclusions and daughter minerals [1-8], the use of polished thin-sections of the original samples is not necessary. Phlogopite and vermiculite samples are presented as large flakes (typically 150x 50 mm) and a thin (ca. 0.1-0.2 mm of thickness) large flake can be easily separated in each case (see experimental), as illustrate in Fig. 1. Thus, the only limitation is the geometry of the interiour chamber of the SEM equipment. For this reason, broken flakes must be used, although the size (ca. 30 x 20 mm) still is considered relatively large. Optical petrographic microscopy observations of phlogopite flakes have allowed to found a variable number and wide distribution of fluid inclusions. Fig. 2 shows, as an example, a typical feature observed directly using a thin flake of this layered silicate material.

In general, the fluid inclusions in phlogopite as observed by SEM were of variable sizes (50-100 μm) and of secondary character because they can be seen with several orientations produced after the crystal growth in accordance with Roedder [3,6]. However, there are daughter minerals crystallized in the phlogopite flakes, as observed by SEM (Fig. 3 a). The cubic morphology is characteristic even at low SEM magnifications. The cubic crystallized formations appeared grouped, being an indication of the existence of fluids during the hydrothermal evolution of the deposit where the samples (phogopite and vermiculite) were found. The EDX analysis (Fig. 3 b) identified these cubic crystals as

Fig. 2 Optical micrograph obtained by petrographic microscopy showingfluid inclusions in a large (ca. 40 x 20 mm) transparent flake (0.2 mm thin) of phlogopite. Magnification 20 X.



NaCl (halite). Morevover, the average elemental ratio as determined in a further EDX study was Na ~ 0.98 and Cl ~ 1.00 and thus confirmed the identification of this mineral phase.

a) b)

Fig. 3 SEM micrograph of crystals observed in a large and thin phlogopite flake (a) and the corresponding EDX analysis of these daughter minerals (b). Fig. 4 shows selected SEM micrographs at higher magnifications, as compared with that presented in Fig. 3, of daughter minerals in fluid inclusions as observed in representative phlogopite large flakes. Cubic crystals have been identified by EDX (Fig. 3d) as NaCl (halite). It can be observed the characteristic cubic form of NaCl inclusion on the phlogopite flake, crystal intergrowths and macles (Fig. 4 a and b), and some non-crystalline, amorphous and rounded forms (Fig. 4 c). Smaller crystals with rounded morphologies and sizes 3-15 μm, even as dendritic forms (Fig. 4b) have been identified by EDX as KCl (sylvite) according to elemental K and Cl. Moreover, it has been also observed cubic crystals of NaCl and KCl mixed as daughter minerals in some phlogopite flakes. Fig. 4 e shows and example of EDX analysis of crystals observed in Fig. 4 b and Fig. 4 c. The mixture of NaCl and KCl besides the main elements present in phlogopite (Si, Al, Mg, Fe) can be thus demonstrated. It should be noted that halite (NaCl) and sylvite (KCl) are common daughter minerals in hypersaline fluid inclusions [3,4,6], which is an indication of the formation and geology evolution of the Santa Olalla deposit [14,15,19]. Further discussion in a next section will be interesting on this subject.

a) b) c)

d) e)

Fig. 4 Selected SEM micrographs of typical morphologies observed in large phlogopite flakes: (a) cubic crystals with macles; (b) cubic crystals forming dendrites with macles; (c) non-crystalline, amorphous and rounded forms. The corresponding chemical analyses by EDX are as follows: the EDX analysis of micrograph (a) is (d). The EDX analysis of (b) is (e) and the EDX analysis of (c) is (f). The effect of the incident electronic beam in thin phlogopite large flakes is not negligible and relatively rapid heating effects, although anisotropic [20,21], are produced. This effect influences the crystallization of the saline fluids in the inclusion and daughter minerals under SEM examination. To illustrate this, Fig. 5 shows a selected SEM micrograph of a rounded inclusion (see Fig. 2) with 60-70 μm diameter, as identified in a large phogopite flake. The corresponding EDX analysis is included in Figure 5c. Inside this fluid inclusion, it can be observed by SEM small crystallite forms. When the incident electronic beam is hold on the phlogopite flake, after a few minutes the heating effect produced a surface break and the explosion of the fluid. The rapid SEM examination inside this inclusion (and the micrograph of



Fig. 5 b is a proof) shows dendrites of small crystals identified as KCl (sylvite), according to the EDX analysis (Fig. 5 d). Note the increase of elemental K and Cl in this analysis as compared with EDX of Fig. 5 c.

a) b)

c) d)

Fig. 5 SEM micrographs of a fluid inclusion observed in a large phlogopite flake (a) and the same after a few minutes under the incident electronic beam (b). The corresponding EDX analyses of each one are included in (c) and (d). Identification of possible presence of potassium-iron chlorides and KCl (sylvite) is difficult [4,6]. According to the phlogopite mineral composition (see experimental) with Si, Al, Mg and Fe (Fig. 5 c) and the presence of cubic crystals, it can be assumed that all the K and Cl elemental composition of EDX spectra is associated to the presence of sylvite (KCl). In fact, Anthony et al. [4], in their study of identification of daughter minerals in quartz, pointed out that difficulties were encountered in dealing with clusters of small minerals because it was difficult to determine what X-rays radiation was contributed by each mineral. These authors suggested that, for example, EDX spectra showing Fe, Na, K and Cl peaks were, in some instances, derived cumulatively from a cluster of NaCl (halite), KCl (sylvite) and iron oxide daughter minerals. Thus, observation of the crystal morphology, such as in the present study (Fig. 5 c and d) often helps to unravel complex EDX spectra. The same authors indicated that salts deposited onto the surface of daughter minerals by evaporation when the inclusion is opened can also contribute spurious radiation to analyses, although minimal effects can be attained [4]. Analysis of daughter minerals by SEM-EDX have been essentially qualitative because quantitative analysis is difficult [6]. It is due to interference from coatings and adjacent phases and for non-uniform geometry, although Anthony et al. [4] have shown that by use of absorption and fluorescence corrections, fairly precise stoichiometry can be obtained for many daughter mineral phases yielding unambiquous identication.

3.2 Vermiculite

Concerning the other layered silicate material, vermiculite, fluid inclusions with daughter minerals were also detected after examination of large transparent flakes by optical microscopy and SEM. It should be noted the effect of the incident electronic beam on thin large flakes of vermiculite, as in the case of phlogopite as described above (Fig. 5). As examples of illustrate this, Figure 6 shows selected SEM micrographs of fluid inclusions and daughter crystallized minerals using high magnifications. They appear sometimes in the interiour of opened and broken large vermiculite flakes as a perfectly broken sheet (Fig. 6 a), in like-triple points (Fig. 6 b) and near cracks along the surface of expanded vermiculite flakes (Fig. 6 c).



a) b) c)

Fig. 6 Selected SEM micrographs (a-c) of large sample flakes of vermiculite (see Fig. 1 b y c) showing evidences of fractured fluid inclusions and crystallized daughter minerals, with damages in zones of the large flake under the action of the incident electronic beam. As observed in phlogopite (Figs. 3 to 5), daughter minerals near fluid inclusions have been observed by SEM in large vermiculite flakes although there are some similarities and differences, as follows. Daughter minerals appeared in cubic crystal forms, although prismatic and acicular or fibrous forms were also typical morphologies observed by SEM (Figs. 6 and 7). Figure 7 shows selected SEM micrographs and the corrresponding EDX analysis. Cubic crystals (Fig. 7 a) have been identified by EDX analysis (Fig. 7 d) as NaCl (halite). In this particular SEM study, cubic macled crystals were not observed in a large relative proportion as compared with that found in phlogopite (Fig. 4). Prismatic crystals observed in vermiculite thin flakes (Fig. 7 b) have identified by EDX (Fig. 7 e) as magnesium dichloride, an important difference with that found studying phlogopite. It should be noted the observations by SEM of cubic and prismatic daughter crystallized in vermiculite although at higher magnifications than in phlogopite (Fig. 4). Finally, it is remarkable that acicular or fibrous crystals have been observed in large thin vermiculite flakes. They have been analyzed by EDX (Fig. 7 f). The detection of Sulphur (S) and Calcium (Ca) suggested that the acicular and/or fibrous crystals can be identified as calcium sulphate (gypsum and/or anhidrite). These crystal formations appeared sometimes mixed with Magnesium and/or Calcium chlorides according to additional evidences by EDX.

a) b) c)

d) e) f)

Fig. 7 Selected SEM micrographs of typical morphologies observed in large vermiculite flakes: (a) cubic crystals; (b) prismatic crystals; (c) acicular and/or fibrous crystal forms. The corresponding chemical analyses by EDX are as follows: the EDX analysis of micrograph (a) is (d). The EDX analysis of (b) is (e) and the EDX analysis of (c) is (f). The EDX analysis of calcium sulphate as found in this study of daughter minerals in vermiculite is merely of further discussion. It should be considered that gold coating by sputtering of large and thin vermiculite samples to be observed by SEM provides better resolution and is more frequently used in SEM studies [2,4,22-24]. The AuMα peak (2.123 keV) from the coating and the SKα peak (2.307 keV) from the sulphate daughter minerals overlap substantially. However, the conditions used for EDX analysis in the present study and the detected amount of calcium sulphate produced a difference in overlapping (Fig. 7 f). It is also important the observations of morphologies by SEM in the study of daughter minerals, in particular in the case of calcium sulphate [4,12]. For instance, Anthony et al. [4] observed by SEM a calcium sulphate with apparent ortorhombic habit in a study of quartz samples known to contain fluid



inclusions having daughter minerals. They assumed from SEM morphology and EDX analysis (Ca, S) to be anhydrite. It is clear that EDX analysis alone cannot resolve whether the analyzed crystals observed by SEM are gypsum, i.e. calcium sulphate dihydrate or gypsum dihydrate, bassanite, anhydrite or even hemihydrite, and only by other additional techniques it is possible [25,26]. In this sense, even the precipitation mechanisms of calcium sulphate mineral phases (gypsum, bassanite and anhydrite) remain large unexplored. Recently, Van Driessche et al. [26] have studied the role and implications of bassanite as a stable precursor phase to gypsum precipitation using time-resolved sample quenching and high-resolution transmission electron microscopy (HR-TEM), providing interesting results to natural and industrial processes.

3.3 Crystallization of daughter minerals in fluid inclusions at the deposit

The above results by optical microscopy and SEM-EDX on phlogopite and vermiculite samples from the Santa Olalla deposit (Huelva, SW Spain), have allowed to found crystallized daughter minerals in fluid inclusions in large thin phlogopite and vermiculite flakes. The identification of crystallized phases has been attained according to morphology and EDX analysis, as follows: (1) halite and sylvite, in phlogopite; (2) halite, possibly sylvite, magnesium dichloride and calcium sulphate, in vermiculite. Identification of possible presence of iron oxides and potassium-iron chlorides, in both cases, is difficult. Taking into account these results, it is clear that halite and sylvite represented the existence of an emission of aqueous saline fluids, mainly of alkaline elements (sodium and potassium), in the formation and geology evolution of this deposit. In fact, halite and sylvite are common daughter minerals found in hipersaline fluid inclusions [3,4]. Furthermore, these fluids must be related to hydrothermal circulation processes in the formation of Santa Olalla skarn better than the meteoric erosion early proposed [14,15], being the origin of both phlogopite and vermiculite layered silicate materials. However, the presence of both magnesium dichloride and halite, as identified as daughter minerals in fluid inclusions in vermiculite (Fig. 7), could also demonstrate the existence of aqueous hydrothermal fluids post-developed after the formation of magnesium aposkarn of Santa Olalla. It is difficult, and even very complex, to know the possible origin of daughter minerals in fluid inclusions as identified here in both layered silicates phlogopite and vermiculite. If a thermodynamic approach is considered [27,28], the phase equilibrium diagrams are a good approach. First of all, the phase equilibrium diagram NaCl-KCl-H2O as proposed by Roedder [27], indicates the formation of continued solid-solution between NaCl and KCl. For this reason, cubic crystals of mixed halite and sylvite have been observed in phlogopite (Fig. 4). This equilibrium phase diagram indicates also the existence of hydrated crystals, such as NaCl•2H2O (hydrohalite), and the formation of an eutectic of NaCl + KCl at 658 ºC. The temperature influences the formation of solid phase minerals. For instance, at room temperature (25 ºC) there is a crystallized phase and liquid according to the composition NaCl + KCl + liquid. Presumablely, the crystals precipitated in separate phases during the cooling taking into account the geology time. This diagram can be applied to any composition of fluid inclusion and daughter minerals containing water with sodium, potassium and chloride ions, showing isotherms and isobars to predict the equilibrium composition and pressure and temperature conditions. On the other hand, from this approach it is possible to estimate the pressures of formation (entrapment) of fluid inclusions, for instance of halite [27,28] , being higher than 0.5 Kbar. They precipitated during cooling from the high temperatures of fluid entrapment. The problem is more complex to be studied using phase equilibrium diagrams when the MgCl2-H2O system is considered in the presence of NaCl and KCl and the origin of calcium sulphate minerals, which have been also identified in the present SEM-EDX study. Additional information by microthermometry of fluid inclusions [3,6] will provide the most reliable information on the characteristic of ore-forming fluids, including the temperature and pressure of the fluid, as well as compositional information such as salinity, metal concentration and gas fugacities. Although the analysis of fluid inclusions and their daughter minerals is important to understand the geological processes that have taken place in the rock and minerals host, the microscopy methods are a complementary technique. A further geology study of the deposit, using additional samples and techniques to provide results on mineral relicts (for instance other calcium minerals, such as tremolite amphybols), altered minerals, siliceous intercalations and isotopic oxygen determinations for geothermometry, would be of interest. The application of SEM-EDX as additional and complementary technique in part of this wide study will be also very valuable.

4. Conclusions

In conclusion, the present study has demonstrated the feasibility of SEM-EDX investigation applied to the study of daughter minerals in fluid inclusions as identified previously by optical microscopy in layered silicate materials, such as phlogopite and vermiculite. Polished thin sections were not used in this investigation, an important difference with precedent studies on daughter minerals in fluid inclusions as found in other minerals, because phogopite and vermiculite are presented as large thin flakes. Fluid inclusions in minerals, generally smaller than 100 μm in diameter, are commonly observed by optical



microscope if trapped in transparent minerals. This is the case of both layered silicates phlogopite and vermiculite. Daughter minerals are found in their opened fluid inclusions. However, the information provided by SEM concerning crystal morphologies and the chemical analysis (qualitative or semiquantitative) by EDX to identify the possible mineral phases, are interesting and very important in this kind of investigations of natural materials. Studies of fluid inclusions and daughter minerals in layered silicate materials have been very scarce and little information by SEM-EDX has can be found in the literature. Thus, the present study is the first contribution to this field considering both layered silicate materials phlogopite and vermiculite at the same deposit (Santa Olalla deposit, Huelva, SW Spain). Cubic crystals in phlogopite, as observed by SEM and analyzed by EDX in solid inclusions of variable sizes (50-100 μm), have been identified as halite (NaCl). Other smaller crystals, with rounded SEM morphologies and sizes 3-15 μm, even as dendritic form, have been identified by EDX as sylvite. It has been also observed and analyzed mixed halite and sylvite in some phlogopite flakes. The effect of the incident electronic beam in thin phlogopite flakes is not negligible and rapid heating effects are produced which influenced the crystallization of the fluid inclusion. Fluid inclusions appear in vermiculite flakes after examination by SEM with daughter minerals showing cubic, prismatic and acicular or fibrous forms. They appear sometimes in the interior of opened and broken vermiculite flakes, in like-triple points and/or surface expanded vermiculite flakes. Cubic crystals observed by SEM in vermiculite have been identified by EDX analysis as halite, and prismatic crystals as magnesium dichloride. Acicular and fibrous crystals as observed by SEM in vermiculite flakes have been also analyzed by EDX, being identified as calcium sulphate. It appears sometimes mixed with Ca and/or Mg chlorides according to EDX results. However, the calcium sulphate mineral phase (gypsum, bassanite or anhydrite or even mixed) is difficult to be precised. Significant differences between the daughter minerals observed by SEM and analyzed by EDX in both layered silicates have been found in despite of the same geological origin. Additional research will be very valuable to understand the formation of phlogopite and vermiculite at the deposit and thus the origin of daughter minerals and fluid inclusions.

Acknowledgements The financial support of Junta de Andalucía to Research Group TEP 204 is acknowledged. The authors want to dedicate this work to the memory of Dr. Guillermo García Ramos, Research Scientist of CSIC who passed away last year, for his previous study and pioneering research on the deposit of phlogopite and vermiculite of Santa Olalla.

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