The origin of large gypsum crystals in the Geode of Pulpí (Almería, Spain)

Abstract

The Geode of Pulpí (Almería, Spain) is an ∼11 m3 ovoid cavity, the walls of which are covered with meter-sized idiomorphic and highly transparent gypsum (CaSO4●2H2O) crystals. We performed a thorough study based on field work, and petrographic and geochemical data collection, which aimed to reconstruct the geological history leading to the formation of this geode. The geode is hosted in mineralized Triassic carbonate rocks with a discontinuous mineral sequence from iron-carbonates and barite to celestine and finally gypsum (microcrystalline and selenite). Data from fluid inclusions show that barite precipitated above 100 °C, celestine at ∼70 °C, and gypsum below 25 °C. All δ34S sulfate phases fall between Triassic and Tertiary evaporite values. Barite and gypsum, either microcrystalline or large selenite crystals, show variable δ34S and δ18O compositions, whereas celestine and centimetric selenite gypsum have homogeneous values. We propose that the growth of the large selenite crystals in the Geode of Pulpí was the result of a self-feeding mechanism consisting of isovolumetric anhydrite replacement by gypsum at a temperature of 20 ± 5 °C, episodically contributed by a ripening process enhanced by temperature oscillations due to climatic change.

Electron transparent nanotubes reveal crystallization pathways in confinement

“…Membranes containing bassanite crystals were left in contact with the mineralization solution for 1 month, after which time all intra-membrane crystals had dissolved. This may be due to Ostwald ripening 38 of larger, unconfined crystals that deposit in the U-tube arms and then grow at the expense of the smaller intra-membrane crystals. 25–100 nm diameter nanotubes containing bassanite crystals were also isolated, transferred to TEM grids, and were imaged before and after storing the grids under ambient conditions for 3 months.…”

“…30,31,47 If a crystal is strongly anisotropic, nuclei that are orientated with their fast growing axis parallel to the long axis of the pore will grow unimpeded at the expense of crystals in other orientations via Ostwald ripening. 38 However, as orientated gypsum and bassanite are observed at early reaction times when few crystals are present, this suggests that competitive growth is unlikely to be active in the membrane pores. The pore surface is therefore likely to be responsible for orienting the crystals in connement.…”

The effect of alumina addition on the synthesis of zeolite from fly ash by hydrothermal method

“…Therefore, to reduce the CaSO4 content, a second purification was carried out using cold water. At low temperatures, the solubility of CaSO4 is greater [10]. Figure 1 shows a decrease in the absorption peak of 25.515 between purification 1 and purification 2 which was caused by CaSO4.…”

101 contact twins in gypsum experimentally obtained from calcium carbonate enriched solutions: mineralogical implications for natural gypsum deposits

“…In marine evaporites, mostly twinned and tabular gypsum crystals are found (Ortı ´, 2011), whereas the most spectacular habits are related to the anhydrite-gypsum thermally driven transformation by a self-feeding mechanism in low supersaturated solutions. This latter condition induces the precipitation of the prismatic metre-sized single crystals and twins of the Naica Mine (Mexico) (Garcı ´a-Ruiz et al, 2007;Ota ´lora & Garcı ´a-Ruiz, 2014) and in the Geode of Pulpı `(Almerı ´a, Spain) (Canals et al, 2019).…”