The Characterization of Natural Gemstones Using Non-Invasive FT-IR Spectroscopy: New Data on Tourmalines
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Fourteen samples of tourmaline from the Real Museo Mineralogico of Federico II University (Naples) have been characterized through multi-methodological investigations (EMPA-WDS, SEM-EDS, LA-ICP-MS, and FT-IR spectroscopy). The samples show different size, morphology and color, and are often associated with other minerals. Data on major and minor elements allowed to identify and classify tourmalines as follows: elbaites, tsilaisite, schorl, dravites, uvites and rossmanite. Non-invasive, non-destructive FT-IR and in-situ analyses were carried out on the same samples to validate this chemically-based identification and classification. The results of this research show that a complete characterization of this mineral species, usually time-consuming and expensive, can be successfully achieved through non-destructive FT-IR technique, thus representing a reliable tool for a fast classification extremely useful to plan further analytical strategies, as well as to support gemological appraisals.
Introduction
The use of non-invasive and non-destructive spectroscopic techniques, such as Fourier Transform Infrared Spectroscopy (FT-IR) and RAMAN, for the characterization of geomaterials of museum interest (ceramics, gemstones, metallic artefacts, bones, etc.) for which a conservative treatment of the sample is required, is gaining more and more interest in applied mineralogy. Demand by stakeholders accounts for reliable, fast and cheap information concerning the structural classification of gemstones (typology, defects, etc.) as well as provenance. However, some uncertainties go along with this kind of applied research, such as the difficulty in fostering new running protocols with non-conventional techniques properly validated by means of a comparative approach using conventional techniques. Actually, it is well-known that this process, in most cases, does not provide more detailed data on a specific geomaterial, however, it may represent a solid guarantee on which the new methodological protocol should be based.
In this frame the Raman spectroscopy and Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFT and variants) nowadays represent the elective analytical techniques due to two main factors: i) extreme optimization of acquisition time ii) in-situ acquisition by properly equipped portable instruments.
The present study, framed within a general survey on characterization of historical samples of the Real Museo Mineralogico of Federico II University of Naples was carried out on tourmalines coming from Europe, Brazil and USA. The choice of tourmalines was based on the premise that they represent a super group consisting of 18 species, all precious gemstones, thus being a significant test of a mineralogical classification only based on FT-IR spectroscopy.
A preliminary chemical characterization by means of different techniques (Scanning Electron Microscopy-Energy Dispersive Spectroscopy-SEM-EDS, Electron Microprobe Analysis-Wavelength Dispersive Spectroscopy-EMPA-WDS, Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry LA-ICP-MS) allowed classifying the investigated tourmalines. Actually, such chemical-based classification was propaedeutic to a spectroscopic analytical approach (external reflectance FT-IR). Main purpose was to evaluate the occurrence of spectroscopic features univocally associated to the mineralogical species previously identified on chemical basis.
Van Hinsberg et al defines tourmaline as "nature's perfect forensic mineral" because "from a single grain, the full geological past of its host rock can be reconstructed, including the pressure–temperature path it has taken through the Earth and the changing fluid compositions it has encountered".
Tourmaline Supergroup Minerals
Tourmaline supergroup minerals have a very complex chemical composition, they are crystallographically acentric borosilicate minerals with the following general chemical formula: XY 3Z 6T 6O18(BO3)3V 3W
where X = □, Na, K, Ca, Pb2+; Y = Li, Mg, Fe2+, Mn2+, Cu2+, Al, V3+, Cr3+, Fe3+, Mn3+, Ti4+; Z = Mg, Fe2+, Al, V3+, Cr3+, Fe3+; T = Si, B, Al; B = B; V = OH, O; W = OH-, F-, O2-
Henry et al. defined the nomenclature of tourmaline species "…in accordance with the dominant-valency rule such that in a relevant site the dominant ion of the dominant valence state". The primary subdivision involves variation at the X site, resulting in the X-site vacant, alkali, and calcic groups. For each group, the occupancy of the Y site is considered in terms of the constituents YFe, YMg, and YLi, recently also YLi, YFe and YMn. If YFe, YMg, and YLi are present root names such as elbaite, dravite, etc. are assigned. Then, three distinct anions (OH-, F-, and O2-) can occur at W site, and their occupancy forms the basis for a general series of tourmaline species: hydroxy-, fluor- and oxy-species. Variation in Z site constituents is expressed in terms of prefixes.
The symmetry of tourmaline is predominantly rhombohedral in the R3m space group (orthorhombic, monoclinic or triclinic symmetry are also present) [35]. In tourmaline one tetrahedrally coordinated site labelled T is predominantly occupied by Si; Al and B can also occur. Six TO4 tetrahedra join corners to make a [T6O18] ring, forming a cyclosilicate structure. There is one triangularly coordinated site that is fully occupied by B. There are two octahedrally coordinated sites, labelled Y and Z, respectively. Three Y octahedra share edges to form a [Y3φ13] trimer (φ = unspecified anion). The Z octahedra share edges around the periphery of this trimer, the BO3 group providing additional linkage. The X site is -coordinated and lies out of the plane of the [T6O18] ring.
Tourmaline in Gemology
Tourmaline also is interesting for researches on gemology studies. Their plenty of colors and hardness combine to make tourmalines spectacular gemstones. Long before it was recognized as a new gem, some magnificent tourmaline gemstones had been mounted as jewels for Europe's royalty. Tourmaline crystals are sensitive to physicochemical changes in their growth, therefore frequently they are zoned with extraordinary color zoning. These features depend on compositional variations, involving several changes in chromophore elements.
FT-IR Spectroscopy of Tourmalines
In a traditional Mid-Infrared spectroscopic analysis, most of the IR radiation absorptions by tourmalines are mainly due to stretching and bending vibrations in tetrahedral [T6O18] and triangular [BO3]3 coordination structural sites, showing main absorption bands in the region 1500 and 400 cm−1. These absorption bands seem to be very useful for distinguishing natural tourmaline and respective imitations. In particular, natural tourmalines are well distinguished from other silicates due to the presence of absorption bands related to the IR active modes of [BO3]3 (1400–1200 cm−1). On the other hand, the main diagnostic absorption bands, related to vibrations of hydroxyl and octahedral cations bonds, are usually located in the high wavenumber region, being OH stretching bands very sensitive to crystal structural changes (> 3000 cm−1).
Non-invasive external-reflection infrared spectroscopy accounts for a region at lower wavenumbers (1400–400 cm−1, hereafter fingerprint region) related to the so-called specular reflection that shows the strongest absorption bands, being the tourmaline crystal an optical flat surface. In this region, absorption bands are inverted by the Reststrahlen effect as foreseen by Fresnel's law for silicates and other inorganic salts with an absorption index k >> 1. As a consequence, relative intensities of the hydroxyl absorption bands at higher wavenumbers (4800–3000 cm−1; hereafter hydroxyl vibrations region), associated to the volume or diffuse reflectance, are weaker.
As well-known from literature, OH groups can occupy two different crystallographic positions in the structure of natural tourmalines. In particular: i) OH1 group is located on W-site, at the center of the hexagonal ring where is connected to three Y cations; ii) OH3 group is located in the V-site around the hexagonal pillar, surrounded by one Y and two Z cations.
Normally, OH3 vibration bands occur at lower wavenumbers than OH1 ones [44], assuming that the frequency of hydroxyl vibration bands has an inverse relationship with the sum of charge of the coordinated cations. Furthermore, OH3 group forms hydrogen bonds with the closest O5 atoms, in contrast to OH1 groups which can be considered as an independent entity in the structure of tourmaline because no significant hydrogen bonds may occur. The different nature of the Y and Z cations along with the different cationic associations related to OH groups, account for very heterogeneous IR spectra.
Conclusive Remarks
The gemological collections of the Real Museo Mineralogico of the Federico II University of Naples offered a spectacular chance in providing natural samples to be advantageously used in researches of applied mineralogy, mainly oriented at preserving and protecting the Cultural Heritage. Actually, the present study investigated natural tourmalines from different areas of the planet, and gave back very interesting starting points concerning the use of the FT-IR spectroscopy aimed at correctly characterizing and classifying this precious gemstone.
The results of this research demonstrate that a complete characterization of tourmaline, usually time-consuming and expensive, can be successfully achieved through non-destructive FT-IR technique, thus representing a reliable tool for a fast classification extremely useful to plan further analytical strategies, as well as to support gemological appraisals. The spectroscopic features observed in the fingerprint region, as well as in the hydroxyl vibrations region, were found to be univocally associated to the mineralogical species previously identified on chemical basis, confirming the reliability of this non-invasive approach.
The present study represents a significant step forward in the application of FT-IR spectroscopy for the characterization of tourmalines, a precious gemstone of great interest in the field of Cultural Heritage. The results obtained can be considered as a solid starting point for the development of new analytical protocols aimed at the fast and reliable identification of tourmaline species, as well as the detection of possible treatments or imitations, with the final goal of supporting gemological appraisals and provenance studies.