The
Cave of Giant Crystals of Naica (Chihuahua, Mexico) is a world
geological treasure worth to be preserved. These crystals of up to
12 m in length are made of selenite, the macrocrystalline variety
of gypsum (CaSO4·2H2O). They have grown
for thousands of years until the cave was dried, which allowed the
cave and the crystals to be accessible, but exposed their surfaces
in contact with air. Gypsum crystals are fragile because of their
trend to dehydrate, the possible replacement to CaCO3 upon
reactions with atmospheric CO2 as well as their intrinsic
mechanical properties. Several laboratory experiments, designed to
study the deterioration of selenite crystals under different artificial
atmospheric conditions, are presented. Four atmospheric compositions
rich in CO2, CH4, NO
x
, and air were tested for 1 year at temperatures of 25 and 60 °C
and in either liquid or gaseous environments. The surface evolution
was monitored by optical microscopy, infrared spectrometry, and grazing
incidence X-ray diffraction with two-dimensional detectors. Surface
alteration and dissolution in a water environment were observed in
short exposition times, as well as the formation of bassanite (CaSO4·1/2H2O). Neither anhydrite
nor calcite was detected. The gaseous environment constituted the
most detrimental conditions to the gypsum crystals integrity.