GATHODOLUMINESCENCE INVESTIGATIONS AND TRACE-EI . EMENTANALYSIS OF OUARTZ BY MICRO . PIXE : tMPLtCAilONS FOR DTAFENETTC AND PROVENANCE STUDIES lN SANDSTONE
Hot-cathodoluminescence (CL) microscopy and high-resolution trace-element analyses by Particle lnduced X-ray Emission (micro-PIXE) using the proton microprobe have been applied to samples of quartz arenite in order to obtain information about their diagenetic environment and the provenance of quartz detritus. The occurrence of an intensely zoned orange-brown cemenl generation within quartz overgrowths in quartzitic lenses from the Fontainebleau Sand (Oligocene, Paris Basin) coincides with increased Fe concentrations of up to 192 ppm. ln contrast, the Fe content of a nonluminescing generation within the same overgrowths is significantly lower. This finding suggests that the CL is either activated by the Fe itself or caused by defects induced by the incorporation of Fe into the structure. Detrital grains of quaflz from sandstone samples of different ages and locations (Wiehengebirgsquarzit, Upper Jurassic, northwestem Germany; Fontainebleau Sand, Oligocene, Paris Basin) yield a good correlation between CL color and Fe and Ti concentrations, with Fe (up to 455 ppm) being more abundant in red-browniuminescing grains and Ti (up to 298 ppm) confined to grains with blue-violet luminescence. Ti is universally present in quartz from plutonic and extrusive igneous rocks, but rarely observed in quartz from metamorphic rocks. This supports the general contention that the blue-violefluminescing quartz was formed under higher temperatures than the quartz with brown luminescence, an observation important for studies of sandstone provenance. Conceming the mechanisms of CL activation, these results do not unequivocally prove that the observed colors are activated by the elements sought. Despite the advantages of the microanalytical technique, the observed trace-element contents may still reflect submicroscopic inclusions rather than element incolporation into the structure.
Luminescence Petrography and Spectroscopic Studies of Diagenetic Minerals
: Cathodoluminescence (CL) petrography is now a routine technique that can provide essential information on provenance, growth fabrics, diagenetic textures and mineral zonation, in addition to enabling more precise quantification of constituents and fabrics. Without the support of CL spectroscopy, however, CL petrography can only remain a fabric analysis technique. Although subtle variations in CL colour recorded on film give important information, describing luminescence intensity and colour from a photographic record is a dubious and subjective affair. The actual CL colour is determined by the number and type of emission and quenching centres present. Superposition of several luminescence bands of different intensities can combine to produce an apparent 'colour', shade or hue. The combination of emission spectroscopy, excitation spectroscopy and pulsed beam technique provide quantitative data on the wavelength and intensity of luminescence and the nature of the luminescing centres. CL spectroscopy should become a standard technique used by the luminescence petrographer because it is the only means of recording CL colours and emission intensity objectively and quantitatively, in addition to providing unique information on the nature of luminescence centres.
Hobbes and the Reception of Leviathan
Although there has been much discussion of the reception of Hobbes’s work, Hobbes’s response to his own reception has rarely received much consideration. This article looks at Hobbes’s engagements with his critics. Far from being rearguard actions by a philosopher under siege, or disingenuous attempts to curry favour with those in power, they can be read as moments when Hobbes in fact sought to convert what he saw as the “truth of speculation into the utility of practice.”