Surface impurities on giant gypsum crystals from 'la Cueva de las Espadas' (Cave of Swords), Naica, Mexico

摘要

The Cave of Swords was discovered in 1910 at Naica, Chihuahua, Mexico. During the last century, human presence has changed the microclimate conditions inside this cave, raising the question of whether anthropogenic action resulted in the deterioration of its gypsum single crystals and in the deposition of impurities on their surfaces. The present work provides a detailed characterization of representative samples of this cave and suggests an answer to the origin of the impurities on the surface of these nature-made large crystals. Laboratory and synchrotron characterization techniques are applied. For the first time, the samples single-crystal nature and the fragmentation effect of impurities are characterized. Synchrotron light diffraction measurements performed on a collection of characteristic samples consistently reveal sharp textures, with crystal reciprocal vectors [0,1,0] preferably perpendicular to the samples principal cleavage planes and orientation widths averaging 8 degrees. X-ray diffraction identifies galena, sphalerite, hematite, goethite and cuprite on the crystals' surface. Diffraction results indicate no correlation between the number of phases of impurities present and crystallinity. Micro X-ray fluorescence clarifies the elemental spatial distribution. The correlations between the elemental distributions confirm the phase identification obtained by diffraction. For Mn and Pb, the correlations point to the presence of amorphous oxides. Minor phases' characterization suggests they have been deposited on iron oxy-hydroxide substrates. All the identified phases correspond to minerals that were abundantly present in Naica ore deposit before any anthropogenic activity. The impurities on the surface of gypsum crystals at the Cave of Swords were not produced by human presence.