Revisiting the Chemistry and Crystal Structure of Sveite

Abstract

Abstract Sveite, KAl7(NO3)4(OH)16Cl2·8H2O, is a rare, water-soluble mineral with an undetermined atomic structure. New major, minor, and trace element data for sveite from three localities (Cerro Autana Cave, Venezuela; San Joaquin Valley, USA; Ponta Grossa, Brazil) are reported using modern analytical techniques (inductively coupled plasma-optical emission spectrometry, inductively coupled plasma-mass spectrometry, and ion chromatography). Powder X-ray diffraction, micro-X-ray diffraction, and scanning electron microscopy-energy dispersive X-ray spectroscopy are used to characterize the identity of residues following digestions of different strengths, i.e., ultrapure H2O, 2%, 20%, and concentrated HCl acid. Sveite from Venezuela has approximately 3 wt.% more K and 5 wt.% less Al than reported by Martini (1980), and that from California has 20 wt.% less Al than was reported by Graham et al. (1988). Sveite from California has higher N (∼8 wt.%) and lower K (4.5 wt.%), Al (∼12 wt.%), and Cl (∼0.5 wt.%) contents compared to that from Venezuela (∼6 wt.% N, ∼7 wt.% K, ∼15 wt.% Al, and ∼10 wt.% Cl contents). Sveite from Brazil has extremely low K (<0.1 wt.%) and high N (∼24 wt.%) contents with respect to those from the other localities. To inhibit formation of aluminum chloride on dissolution, which can subsequently impact results from geochemical analyses, it is recommended that ultrapure H2O be used to digest sveite. Geochemical and spectroscopic evidence suggests that sveite from different localities may represent distinct mineral species within a potentially larger group of sveite-type minerals. A revised chemical formula for sveite from Venezuela is K2.73Al7.97(NO3)6O8.27Cl4.1·35H2O, Z = 2, leading to a more general formula of (K3−x□x) Al8(NO3)6O8Cl4·nH2O, Z = 2. The lack of K and Cl in material from other localities that shows similar powder diffraction patterns would require a more general formula.