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The Cave of Swords was discovered in 1910 at Naica mine, Chihuahua, Mexico. Its name refers to the look of the 1-2 m long crystals the cave had when it was discovered. Currently the crystals are 0.1-0.3 m long. The crystals surface is opaque and ocher. For over 100 years these crystals continue to amaze and give us clues about their formation. This work is part of a research aimed at the conservation of the Naica Giant Crystals. Thirteen samples from the Cave of Swords were analyzed by Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), Confocal Laser Microscopy with Differential Interference Contrast (LCM-DIM) and Transmission Electron Microscopy (TEM). X-Ray Fluorescence (μ-XRF) together with X-ray Absorption Near Edge Structure (μ-XANES) and X-ray Photoelectron Spectroscopy (XPS) were employed for elemental analysis. For phase analysis, X-ray diffraction (XRD) in both symmetric and grazing incidence geometries (GI-XRD) and Micro electron diffraction at TEM were used. Impurities on crystals surfaces show a heterogeneous distribution of the present elements. The thickness of impurities ranges from 120 nm to 150 μm. The phases identified were (see figure) gypsum (1, 2, 3, 6, 9, 10, 13), hematite (4, 7, 8), sphalerite (14), chalcopyrite (11), cuprite (15), galena (5), alabandite (12), halite, fluorite and amorphous Pb and Mn oxy-hydroxides. Al, C, Ca, Cl, Cu, F, Fe, Mg, Mn, Na, O, Pb, S, Si and Zn elements were identified. A model for the origin of impurities follows: Selenite stopped growing when the solution became sub-saturated. Then, hematite was deposited as the main phase, which was dissolved or suspended in the solution. Hematite matrix served for the adsorption of other crystalline and amorphous phases. We concluded that humans have not produced the impurities, which are witnesses of the gypsum crystals formation. Acknowledgment: Stanford Synchrotron Radiation Lightsource, Harvard Museum of Natural History and CONACYT CB-183706.
abstracts
The Cave of Swords was discovered in 1910 at Naica mine, Chihuahua, Mexico. Its crystals now are 0.1-0.3m long and their surface is opaque and ocher. For over 100 years these crystals continue to amaze and give us clues about their formation [1]. This work refers to the use of synchrotron radiation for phase identification on gypsum single crystals surfaces (GSCS). The experiments were performed at beamlines (BL) 11-3 and 2-3 of the Stanford Synchrotron Radiation Lightsource (SSRL). Synchrotron X-Ray micro-Fluorescence (μ-SXRF) and micro-X-ray absorption (μ-XANES) at BL 2-3, as well as Grazing Incidence X-ray Diffraction (GI-XRD) and Transmission X-ray Microscope (TXM) at BL 11-3, were employed for elemental and phase identification. For µ-SXRF spectroscopy at the Pb LIII absorption edge some region of interests were selected on each sample. In some samples Fe K-edge μ-XANES spectra were obtained. Spectra from BL 2-3 were analyzed by standard procedures of in-house software tools and using SMAK [2]. Ni, Cu, Co, V, K, Ti, Fe, Mn, Pb, Zn, Ca and S elements were identified. Hematite phase was identified by μ-XANES. All GI-XRD and TXM 2D X-ray diffraction patterns (2D-XRD) were calibrated using standard procedures developed at BL 11-3. 2D-XRD data were analyzed by ANAELU [3], Wfdiff [2] and FindIt codes. Figure shows above a TXM image, mapping where 2D-XRD patterns were recorded. Impurities in the sample increase from left to right, as was observed by direct inspection. Below, 2D-XRD patterns show at left the single crystal spots and at right the mosaic tracks of gypsum crystals. It was concluded that the crystal structure is affected by impurities. Hematite, chalcopyrite, sphalerite, cuprite, galena, and alabandite phases were determined by GI-XRD at GSCS. Acknowledgment: Stanford Synchrotron Radiation Lightsource, Harvard Museum of Natural History and CONACYT CB-183706.
