Naica's "Cueva de los Cristales": Synchrotron radiation characterization of the wall-crystal interface

Naica's "Cueva de los Cristales" was discovered in 2000. It has been considered particularly interesting for its beauty and the challenges it poses to crystallography. This article focuses on the study of the wall-selenite interface by various techniques, particularly X-ray diffraction (XRD), scanning electron microscopy (SEM), with emphasis on micro-X-ray fluorescence (micro-XRF) and micro-X-ray absorption near edge structure (micro-XANES). The main phases calcite, quartz, goethite and montmorillonite were identified by XRD, as well as the association of crystalline and amorphous minor and trace phases of Zn, Mn, Cu, As and Pb. The latter were identified in micro-XRF maps and micro-XANES spectra. The results for the morphology and the chemical description of the crystal-wall interface may contribute to propose a nucleation and growth mechanism for Naica megacrystals

Evolution of the Atonishing Naica Giant Crystals in Chihuahua, Mexico

Calcium sulfate (CaSO4) is one of the most common evaporites found in the earth’s crust. It can be found as four main variations: gypsum (CaSO4·2H2O), bassanite (CaSO4 ·0.5H2O), soluble anhydrite, and insoluble anhydrite (CaSO4), being the key difference the hydration state of the sulfate mineral. Naica giant crystals’ growth starts from a supersaturated solution in a delicate thermodynamic balance close to equilibrium, where gypsum can form nanocrystals able to grow up to 11–12 m long. The growth rates are reported to be as slow as (1.4 ± 0.2) × 10−5 nm/s, taking thousands of years to form crystals with a unique smoothness and diaphaneity, which may or may not include solid or liquid inclusions. Conservation efforts can be traced back to other gypsum structures found prior to Naica’s. Furthermore, in the last two decades, several authors have explored the unique requirements in which these crystals grow, the characterization of their environment and microclimatic conditions, and the prediction of deterioration scenarios. We present a state-of-the-art review on the mentioned topics. Beyond the findings on the origin, in this work we present the current state and the foreseeable future of these astounding crystals.The support provided by CONACYT Project No. 183706, the proposals SSRL 3939, the ESRF HG-77, Elettra Sincrotrone-Trieste 20155328, as well as by the International Centre for Theoretical Physics (ICTP) are acknowledged. Special thanks to Manuel Reyes-Cortés, who provided some of the key research samples and supported their selection and analysis. The authors thank the cooperation of the management of the Peñoles company, the Desert Museum of Ciudad Delicias, Chihuahua, the Harvard Museum of Natural History, and the Faculty of Engineering of the Autonomous University of Chihuahua for providing essential specimens for the study. The authors thank the anonymous referees for their helpful suggestions

Local piezo-response for lead-free Ba0.9Ca0.1Ti0.9Zr0.1O3 electro-ceramic by switching spectroscopy

<div><p>The purpose of this work is to determine the effective piezoelectric coefficient (d33) and the macro ferroelectric hysteresis behavior for the Ba0.9Ca0.1Ti0.9Zr0.1O3 (BCZT). The sample was prepared by the modified Pechini method and it was sintered at 1250 ºC for 5 h. The refinements of X-ray diffraction (XRD) patterns obtained by the Rietveld method suggest a slight degree of tetragonality (c/a = 1.0025) in the perovskite structure. High counting statistics was performed in the two-dimensional grazing incidence 2D-GIXRD characterization by using synchrotron radiation. These results and the Raman spectrum analysis support the XRD interpretation. The morphology reveals a non-homogeneous terrace-type shape with a grain size distribution centered at 13 microns. The switching spectroscopy piezo-response force microscopy was used to obtained the effective d33 = 142 pm/V. The soft ferroelectric hysteresis shows a coercive field Hc = 1.3 kV/cm with a saturation polarization Ps = 15.7 µC/cm2.</p></div