New insight into crystal chemistry of topaz : a multi-methodological study

The crystal chemistry of a natural topaz [with OH/(OH + F) < 0.5] was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, single-crystal X-ray diffraction (at 298 K) and neutron diffraction (at 298 and 10 K), and polarized infrared spectroscopy to define unambiguously the real symmetry of topaz, the location of the proton and its thermal displacement parameters at room and low temperatures, the hydrogen-bonding and the vibration modes (stretching and bending) of the OH dipole. X-ray and neutron structural refinements allow us to infer that the crystal structure of natural topaz with OH/(OH + F) < 0.5 can be described with the Pbnm space group. Violating reflections, found in the previous investigations and in this study, are likely due to Renninger effect (double diffraction phenomenon). The nuclear density Fourier map shows that the proton is located at Wyckoff 8d position and the refined coordinates are: x= 0.495(2), y= 0.252(1), z= 0.1629(7). The O-H bond lies on the (010)-plane and forms an angle of about 28.9 degrees with the c-axis. Neutron structural refinements at 298 and 10 K show that the displacement ellipsoid of the proton is highly anisotropic. The H-bonding arrangement appears to be complex, with at least four potential H center dot center dot center dot O/F interactions (distances < 2.38 A). The topological configuration of the O-H group described by the neutron structural refinements is confirmed by the infrared investigation: the OH stretching mode (at 3640 cm(-1)) has no component of vibration parallel to the b axis (i.e., the O-H direction is perpendicular to [010]). The OH bending mode (at 1161 cm(-1)) shows components along the three crystallographic axes, which appear to be more prominent along the a and b-axes. The possible distribution into the crystal structure of topaz of the minor/trace elements found (Na, Ca, Fe Cr, V, Ti, B), and the implied topological effects, is discussed

Apatite : a new redox proxy for silicic magmas?

The oxidation states of magmas provide valuable information about the release and speciation of volatile elements during volcanic eruptions, metallogenesis, source rock compositions, open system magmatic processes, tectonic settings and potentially titanium (Ti) activity in chemical systems used for Ti-dependent geothermometers and geobarometers. In this paper we explore the use of Mn in apatite as an oxybarometer in intermediate and silicic igneous rocks. Increased Mn concentrations in apatite in granitic rocks from the zoned Criffell granitic pluton (southern Scotland) correlate with decreasing Fe2O3 (Fe3+) and Mn in the whole-rock and likely reflect increased Mn2+/Mn3+ and greater compatibility of Mn2+ relative to Mn3+ in apatite under reduced conditions. Fe3+/Fe2+ ratios in biotites have previously been used to calculate oxygen fugacities (fO2) in the outer zone granodiorites and inner zone granites where redox conditions have been shown to change from close to the magnetite-hematite buffer to close to the nickel-nickel oxide buffer respectively ( Stephens et al., 1985). This trend is apparent in apatite Mn concentrations from a range of intermediate to silicic volcanic rocks that exhibit varying redox states and are shown to vary linearly and negatively with log fO2, such that logfO2=-0.0022(±0.0003)Mn(ppm)-9.75(±0.46)logfO2=-0.0022(±0.0003)Mn(ppm)-9.75(±0.46) Variations in the Mn concentration of apatites appear to be largely independent of differences in the Mn concentration of the melt. Apatite Mn concentrations may therefore provide an independent oxybarometer that is amenable to experimental calibration, with major relevance to studies on detrital mineral suites, particularly those containing a record of early Earth redox conditions, and on the climatic impact of historic volcanic eruptions