Structural breakdown and thermal behavior of selected borosilicate minerals

The specific objective of this thesis was the definition of thermal behavior, breakdown mechanisms and breakdown products of selected borosilicates, specifically tourmalines and axinites, which are the most and the less investigated borosilicate phases, respectively, occurring in geological settings. The general objective was to assess the role of borosilicates in the dehydration embrittlement process along the downgoing slab, leading to the definition of their stability fields and to the quantitative evaluation of B-enriched fluids released during possible oxidation-deprotonation processes and structural breakdown of such mineral phases

In situ high‑temperature behaviour and breakdown conditions of uvite at room pressure

The thermal behaviour of an uvite from San Piero in Campo (Elba Island, Italy) was investigated at room pressure through in situ high-temperature powder X-ray diffraction (PXRD), until the breakdown conditions were reached. The variation of uvite structural parameters (unit-cell parameters and mean bond distances) was monitored together with site occupancies and we observed the thermally induced Fe oxidation process counterbalanced by (OH)− deprotonation, which starts at 450 °C and is completed at 650 °C. The uvite breakdown reaction occurs between 800 and 900 °C. The breakdown products were identified at room temperature by PXRD and the breakdown reaction can be described as follows: tourmaline → indialite + yuanfuliite + plagioclase + “boron-mullite” phase + hematite

HT breakdown of Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar

The thermal behavior of a gem-quality purplish-red Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar, with composition X(Na0.41□0.35Ca0.24)Σ1.00 Y(Al1.81Li1.00Fe3+ 0.04Mn3+ 0.02Mn2+ 0.12Ti0.004)Σ3.00 ZAl6[T(Si5.60B0.40)Σ6.00O18](BO3)3(OH)3 W[(OH)0.50F0.13O0.37]Σ1.00 was investigated using both in situ High-Temperature X-Ray powder diffraction (HT-pXRD) and ex situ X-Ray single-crystal diffraction (SC-XRD) on two single crystals previously heated in the air up to 750 and 850 °C. The first occurrence of mullite diffraction peaks allowed us to constrain the breakdown temperature of Mnbearing elbaite at ambient pressure, at 825 °C. The breakdown products from the HT-pXRD experiments were cooled down to ambient temperature and identified via pXRD, represented by B-mullite and γ-LiAlSi2O6. A thermally induced oxidation of Mn2+ to Mn3+ was observed with both in-situ and ex-situ techniques; it started at 470 °C and is assumed to be counterbalanced by deprotonation, according to the equation: Mn2+ + (OH)– → Mn3+ + O2– + 1/2H2. At temperatures higher than 752 °C, a partial disorder between the Y and Z sites is observed from unit-cell parameters and mean bond distances, possibly caused by the inter-site exchange mechanism YLi + ZAl → ZLi + YAl

Thermal behavior of schorl up to breakdown temperature at room pressure

Schorl is one of the most widespread tourmaline compositions in the world, known from many different geological settings. Its role as boron and water carrier has been moderately investigated together with its stability field. In this study, the richest schorl in Fe2+ content was investigated to constraint its breakdown temperature at room pressure through in situ powder X-Ray Diffraction (in situ pXRD), its breakdown products and the coupled thermally induced dehydrogenation-dehydrogenation process experienced approaching the breakdown conditions. Schorl turned out to begin its breakdown at 850 °C with the first appearance of hematite, followed by a dominant B-mullite phase. The breakdown reaction of schorl can be expressed as follows: 2NaFe2+3Al6(BO3)3Si6O18(OH)=3Fe2O3+4/3Al9Si2BO19+(Na- Si- B-rich) glass+4H2O.The breakdown process is completed at 950 °C, when no trace of residual tourmaline is found. Annealing the schorl at 450 °C in air was enough to set the Fe oxidation out, counterbalanced by the deprotonation reaction: (Fe2+)+(OH)- → (Fe3+)+ (O2-)+1/2H2(g)

Recommended X-ray single-crystal structure refinement and Rietveld refinement procedure for tremolite

A detailed description of the structure of the amphibole-supergroup minerals is very challenging owing to their complex chemical composition that renders the process of cation partition extremely difficult, particularly because of the occurrence of multivalent elements. Since amphiboles naturally occur under a fibrous morphology and have largely been used to produce asbestos, there is a growing demand for detailed and accurate structural data in order to study the relationships between structure, composition and toxicity. The present study proposes a recommended refinement procedure for both X-ray single-crystal structure refinement (SREF) and Rietveld analysis for tremolite, selected as a test case. The corresponding structural results are compared to estimate the ‘degree of confidence’ of the Rietveld refinement with regard to SREF. In particular, it is shown that the interpretation of the electron density of the tremolite structure by SREF is model dependent. By assuming that the sitescattering values from SREF should be as close as possible to those from electron microprobe analysis, as a crucial constraint for the correct description of the final crystal-chemical model, it is found that it is best satisfied by using partially ionized scattering curves (SCs) for O and Si, and neutral SCs (neutral oxygen curves or NOCs) for other atoms. This combination leads to the best fit to the diffraction data. Moreover, it is found that Rietveld refinement using NOCs produces the best structural results, in excellent agreement with SREF. It is worth noting that, due to the complexity of the diffraction pattern and the fairly large number of freely refinable parameters, refinements with different combinations of SCs produce results almost indistinguishable from a statistical point of view, albeit showing significant differences from a structural point of view

Cr diffusion in MgAl2O4 synthetic spinels: preliminary results.

Chromian spinel is an accessory phase common in crustal and mantle rocks, including peridotites, gabbros and basalts. Spinel, it has been identified as one of the most effective, sensible, and versatile petrogenetic indicator in mafic and ultramafic rock systems due to the strict interdependence between its physico-chemical properties (chem- ical composition, cation configuration etc.) and genetic conditions (temperature, pressure, and chemical character- istics of the system). In particular, studies on intra- and inter-crystalline Mg-Fe2+, Cr-Al exchange demonstrated the close relationship between spinel composition and both degree of partial melting and equilibrium temperature of spinel-peridotites. Moreover, studies focused on the chemical zoning of Mg-Fe2+ and/or Cr-Al components in spinel have been used, combined with a diffusion model, to provide quantitative information on peridotites and gabbros pressure-temperature paths and on deformation mechanisms. Although these potentials, most of the experimental studies have been performed on spinels hosting a limited content of divalent iron (sensu stricto, MgAl2O4), whereas the scarce studies on Cr-Al inter-diffusion coefficient have been performed at 3-7 GPa as pressure boundary condition. In order to contribute to the understanding of processes occurring in the lithospheric mantle, we have initiated an experimental research project aiming at determining the Cr-Al inter-diffusion in spinel at 2 GPa pressure and temperature ranging from 1100 to 1250°C. The experiments were performed in a end-loaded piston cylinder by using a 19 mm assembly and graphite-Pt double capsules. As starting materials we used synthetic Mg-Al spinel (200-300 microns in size) and Cr2O3 powder. Microanalyses of experimental charge were performed on polished carbon-coated mounts by electronic microprobe. Line elemental analyses were made perpendicular to the contact surface between Cr2O3 powder and spinel, at interval of 2 microns. By processing these preliminary data, we have estimated a diffusion coefficient of chromium (D) of 7.6*10-15 m2/

Schorl-1A from Langesundsfjord (Norway)

A crystal fragment of schorl from Langesundsfjord (Norway), showing a zonation with a biaxial optic behavior in the rim, was studied by electron microprobe analysis, single-crystal X-ray diffraction, Mössbauer, infrared and optical absorption spectroscopy and optical measurements. Measured 2Vx is 15.6°. We concluded that biaxial character of the sample is not due to internal stress because it cannot be removed by heating and cooling. Diffraction data were refined with a standard R3m space group model, with a = 16.0013(2) Å, c = 7.2263(1) Å, and with a non-conventional triclinic R1 space-group model keeping the same hexagonal triple cell (a = 16.0093(5) Å, b = 16.0042(5) Å, c = 7.2328(2) Å, α = 90.008(3)°, β = 89.856(3)°, γ = 119.90(9)°), yielded Rall = 1.75% (3136 unique reflections) vs. Rall = 2.53% (17342 unique reflections), respectively. The crystal-chemical analysis resulted in the chemical formula X(Na0.98K0.01☐0.01)Σ1.00 Y(Fe2+ 1.53Al0.68Mg0.35Ti0.20Fe3+ 0.20Mn0.02V0.01Zn0.01)Σ3.00 Z(Al5.10Fe2+ 0.50 Mg0.40)Σ6.00(Si6O18)(BO3)3(OH)3[(OH)0.39F0.22O0.39]Σ1.00, which agrees well in terms of calculated site-scattering (X 10.9 epfu, Y 63.7 epfu, Z 83.7 epfu) and refined site-scattering (X 11.4 epfu, Y 63.4 epfu, Z 83.6 epfu). About 0.19 apfu Fe2+ is at the Z sites in the R1 model that showed that one out of six independent Z sites (Zd) has higher refined site scattering [15.5 eps vs. mean 13.7(2) eps for the other five sites] and larger mean bond length [1.969 Å vs. 1.927(6) Å for the other five sites] and larger octahedral angle variance [53° vs. 42(3)°]. All these features support local order of Fe2+ at the Zd site. Optical absorption spectra also show evidence of Fe2+ at the Z sites. The elongation of the Zd-octahedron is along a direction that forms an angle of ca. 73° with a unit-cell edge and is coincident with the direction of the γ-refraction index. All these data support the triclinic character of the structure of the optically biaxial part of the tourmaline sample from Langesundsfjord and provide evidence that even in the presence of excellent statistical agreement factors from excellent X-ray diffraction data, the lowering of symmetry due to cation ordering may have been overlooked in many other tourmaline samples in the absence of a check of the optical behaviour. According to the nomenclature rules, the studied triclinic schorl, should be named schorl-1A

Anortho-schorl from Langesundsfjord (Norway)

Some previous studies report of optically anomalous tourmaline with biaxial character, which is incompatible with its putative R3m symmetry. A crystal fragment of sample GEO-NRM19252409 (Swedish Museum of Natural History) coming from Langesundsfjord (Norway) showing zonation with biaxial optic behavior in the rims was studied by means of electron microprobe, singlecrystal X-ray diffraction, Mössbauer, infrared and optical absorption spectroscopy and optical measurements. The latter were performed with a spindle stage in a Leitz Dialux microscope equipped with a CCD camera using Excelibr spreadsheet (Steven & Gunter, 2018), using a 30 μm thin section of the crystal cut perpendicular to c axis. Conoscopic image showed a biaxial interference figure, with negative optic sign. Measure 2Vx was 15.6° (white light). The biaxial character of the sample is not due to internal stress since it cannot be removed by heating and cooling. Orientation of the optical indicatrix was obtained using the same crystal mounting at a Rigaku XtaLAB Synergy-S diffractometer (MoKα) and showed c^X = 2°, b^Z = 164.3° and a^Z = 75.8°. Complete data collection was obtained on a full sphere up to 0.50 Å. Diffraction data were refined with a standard R3m space group model [a 16.0013(2) Å, c 7.2263(1) Å] and with a nonconventional triclinic R1 space group model from Hughes et al. (2011), keeping the same hexagonal triple cell for comparison purposes but with unconstrained unit-cell parameters: a 16.0093(5) Å, b 16.0042(5) Å, c 7.2328(2) Å, α 90.008(3)°, β 89.856(3)°, γ 119.90(9)°. Trigonal Rint = 6.6%, Rall = 1.75% (3136 unique reflections) vs. triclinic Rint = 4.1%, Rall = 2.53% (17342 unique reflections). In both structure models, protons bonded to O3 sites were located and their coordinates refined. No proton was observed close to O1, in agreement with chemistry. Crystal- chemical analysis resulted in the chemical formula (Na0.98K0.010.01)Σ1.00 Y(Fe2+ 1.73Mg0.12Al0.90Ti0.21Mn0.02V0.0 1Zn0.01)Σ3.00 Z(Al4.88Fe3+ 0.30Fe2+ 0.19Mg0.63)Σ6.00(Si6O18)(BO3) (OH)3[(OH)0.29F0.22O0.49]Σ1.00, which agrees well in terms of calculated site scattering (X 10.9 epfu, Y 63.7 epfu, Z 83.7 epfu) and refined site scattering (X 11.4 epfu, Y 63.4 epfu, Z 83.6 epfu). The ordering of about 0.19 apfu of Fe2+ at the Z sites comes from the structure refinement of the R1 model that showed that one out of 6 independent Z sites (Zd site) shows higher refined site scattering (15.5 eps vs. mean 13.3 eps for the other 5 sites) as well as larger mean bond length (1.968 Å vs. 1.925, s.d. 0.006, Å for the other 5 sites) and larger octahedral angle variance (53° vs. 42°, s.d. 3). All these features support the local ordering of Fe2+ at the Zd site. A less pronounced ordering of Al is also observed at the Yc site (lower refined scattering and smaller mean bond length than the other two Y sites), which shares one edge with the Zd site. Optical absorption spectra also show evidence of Fe2+ at the Z sites. Interestingly, the elongation of the Zd-octahedron is along a direction which forms an angle of ca. 73° with a unit-cell parameter and is coincident with the direction for γ-refraction index. All these evidences support the triclinic character of the tourmaline structure from Langesundsfjord, and put on evidence that even in the presence of excellent statistical residual factors from excellent X-ray diffraction data, the lowering of symmetry due to cation ordering may have been overseen in many other tourmaline samples in absence of an opportune check of the optical behavior. Although the symmetry of the present sample is not trigonal, the basic structure type of tourmaline is retained. Thus, it can be classified as the triclinic dimorph of schorl

Mn-bearing purplish-red tourmaline from the Anjanabonoina pegmatite, Madagascar

A gem-quality purplish-red tourmaline sample of alleged liddicoatitic composition from the Anjanabonoina pegmatite, Madagascar, hasbeen fully characterised using a multi-analytical approach to define its crystal-chemical identity. Single-crystal X-ray diffraction, chem-ical and spectroscopic analysis resulted in the formula: X(Na0.41□0.35Ca0.24)Σ1.00Y(Al1.81Li1.00 Fe3+0.04Mn3+0.02Mn2+0.12Ti0.004)Σ3.00 ZAl6[T(Si5.60B0.40)Σ6.00O18] (BO3)3(OH)3W[(OH)0.50F0.13O0.37]Σ1.00, which corresponds to the tourmaline species elbaite having the typical space group R3m and relatively small unit-cell dimensions, a= 15.7935(4) Å, c= 7.0860(2) Å and V= 7.0860(2) Å3.Optical absorption spectroscopy showed that the purplish-red colour is caused by minor amounts of Mn3+(Mn2O3= 0.20 wt.%).Thermal treatment in air up to 750°C strongly intensified the colour of the sample due to the oxidation of all Mn2+ to Mn3+ (Mn2O3 up to 1.21 wt.%). Based on infrared and Raman data, a crystal-chemical model regarding the electrostatic interaction betweenthe X cation and W anion, and involving the Y cations as well, is proposed to explain the absence or rarity of the mineral species ‘liddicoatite’

Crystal‑chemical behavior of Fe2+ in tourmaline dictated by structural stability: insights from a schorl with formula NaY(Fe2+2Al)Z(Al5Fe2+)(Si6O18)(BO3)3(OH)3(OH,F) from Seagull batholith (Yukon Territory, Canada)

A black tourmaline sample from Seagull batholith (Yukon Territory, Canada) was established to be a schorl with concentrations of Fe2+ among the highest currently found in nature (FeOtot ~ 18 wt.% and Fe2+ ~ 100% of Fetot) on the basis of a multi-analytical characterization through Mössbauer spectroscopy, electron microprobe, Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry and single-crystal X-ray diffraction. From the crystal-chemical analysis, the following empirical formula is proposed: X(Na0.74□0.24K0.01Ca0.01)Σ1.00Y(Fe2+2.05Al0.92Ti0.02Mn0.01Zn0.01)Σ3.00Z(Al5.41Fe2+0.53Mg0.06)Σ6.00(Si6O18)(BO3)3V(OH)3W[(OH)0.46F0.41O0.13]Σ1.00, which can be approximated as NaY(Fe2+2Al)Z(Al5Fe2+)(Si6O18)(BO3)3(OH)3(OH,F). Compared to the formula of the ideal ordered schorl, NaY(Fe2+3)Z(Al6)(Si6O18)(BO3)3(OH)3(OH), the studied sample has a partial disorder of Fe2+ across the Y and the Z sites that can be expressed by the intracrystalline order–disorder reaction YAl + ZFe2+ → YFe2+ + ZAl. Such a partial cation disorder must be invoked to explain tourmaline structural stability because an ideal ordered schorl results in a large misfit between the < YFe2+–O > and < ZAl3+–O > mean bond lengths (that is, between the YO6 and ZO6 polyhedra). This misfit is reduced by introducing Al at Y (i.e., through the < Y–O > shortening) and Fe2+ at Z (i.e., through the < Z–O > lengthening). The result is that in tourmaline the site distribution of high Fe2+ concentrations is dictated by long-range structural constraints