A Mineralogical Study of a Dravitic Tourmaline from the Grenville Province

Tourmaline from a gem-quality deposit in the Grenville province has been studied with X-ray diffraction, visible-near infrared spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, electron microprobe and optical measurements. The tourmaline is found within tremolite-rich calc-silicate pods hosted in marble of the Central Metasedimentary Belt. The crystals are greenish-greyish-brown and have yielded facetable material up to 2.09 carats in size. Using the classification of Henry et al. 2011 the tourmaline is classified as a dravite, with a representative formula shown to be (Na0.73Ca0.2380.032)(Mg2+2.913Fe2+0.057Ti4+0.030) (Al3+5.787Fe3+0.017Mg2+0.14)(Si6.013O18)(BO3)3(OH)3((OH,O)0.907F0.093). Rietveld analysis of powder diffraction data gives a = 15.9436(8) Å, c = 7.2126(7) Å and a unit cell volume of 1587.8 Å3. A polished thin section was cut perpendicular to the c-axis of one tourmaline crystal, which showed zoning from a dark brown core into a lighter rim into a thin darker rim and back into lighter zonation. Through the geochemical data, three key stages of crystal growth can be seen within this thin section. The first is the core stage which occurs from the dark core to the first colourless zone; the second is from this colourless zone increasing in brown colour to the outer limit before a sudden absence of colour is noted; the third is a sharp change from the end of the second and is entirely colourless. These events are the result of metamorphism and hydrothermal fluids resulting from nearby felsic intrusive plutons. Scanning electron microscope, and electron microprobe traverses across this cross-section revealed that the green colour is the result of iron present throughout the system while the brown colour is correlated with titanium content. Crystal inclusions in the tourmaline of chlorapatite, and zircon were identified by petrographic analysis and confirmed using scanning electron microscope data and occur within the third stage of formation.

CHARACTERIZATION OF FILLING MATERIALS FOR EASEL PAINTINGS

Developing appropriate treatments for easel paintings can be complex, as many works are composed of various materials that respond in different ways. When selecting a filling material for these artworks, several properties are investigated including: the need for the infill to react to environmental conditions in a similar manner as the original material; the need for the infill to have good handling properties, adhesion to the original support, and cohesion within the filling material; the ability for the infill to withstand the stress of the surrounding material and; be as flexible as the original material to not cause further damage. Also, changes in colour or mechanical properties should not occur as part of the ageing process. Studies are needed on acrylic-based materials used as infills in conservation treatments. This research examines some of the chemical, physical, and optical changes of eleven filling materials before and after ageing, with the aim to evaluate the overall appropriateness of these materials as infills for easel paintings. The materials examined were three rabbit skin glue (RSG) gessoes, and seven commercially prepared acrylic materials, all easily acquired in North America. Chemical analysis was carried out with Fourier transform infrared (FTIR) spectroscopy and X-ray fluorescence (XRF), pyrolysis gas chromatography-mass spectroscopy (Py-GC/MS), and differential scanning calorimetry (DSC). Overall the compositions of the various materials examined were found to be in agreement with the available literature and previous research. This study also examined characteristics of these materials not described in previous works and, additionally, presented the compositions and behaviour of several commonly used materials with little literature description. After application of an ageing regimen, most naturally aged and artificially aged samples displayed small changes in gloss, colour, thickness, and diffusive behaviour; however, to evaluate these materials fully mechanical testing and environmental studies should be carried out.