On the crystal chemistry of sulfur-rich lazurite, ideally Na7Ca(Al6Si6O24)(SO4)(S3)–·nH2O

Abstract

Dark blue lazurite from the Malo-Bystrinskoe lazurite deposit, Baikal Lake area, Eastern Siberian region, Russia, was analyzed by electron microprobe and revealed an unusually high content of total sulfur corresponding to 8.3 wt% S. The relative content of sulfur in sulfate and sulfur in sulfide form was determined by wet chemical analysis. The H2O content was measured by means of differential thermal analysis in combination with mass spectrometry and infrared (IR) spectroscopy. The charge-balanced empirical formula of lazurite calculated on the basis of 12 (Al+Si) atoms per formula unit was (Na6.97Ca0.88K0.10)Σ7.96[(Al5.96Si6.04)Σ12O24](SO4)1.092−(S3−)0.55S0.052− Cl0.04·0.72H2O. The presence of H2O molecules and (S3)– and (SO4)2– groups was confirmed by the combination of IR, Raman, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) methods. The idealized formula of lazurite is Na7Ca[Al6Si6O24](SO4)2–(S3)–·H2O, and it is believed that extra-framework cations and anions are grouped into clusters of [Na3Ca·SO4]3+ and [Na4(S3)–]3+. The types of isomorphous substitutions in nosean and haüyne are discussed. Lazurite is a clathrate-type mineral, which may be an effective (S3)– sensor due to the stability of the trisulfur radical anion in isolated cages of the crystal structure. This specific feature makes it possible to study the behavior of this ubiquitous radical anion over larger T and P ranges as compared to free species. This kind of lazurite, with oxidized and reduced sulfur species, seems to be appropriate for the estimation of the fugacity of SO2 and O2 in metasomatic systems forming lazurite-containing rocks. The systematic presence of incommensurate modulations is a unique structural feature of Baikal lazurite and may be an important marker indicating provenance of the mineral.

Spectroscopic and Crystal-Chemical Features of Sodalite-Group Minerals from Gem Lazurite Deposits

“…•− -rich lazurite ZK (earlier described in detail in [8]), grains of forsterite and pyroxene are corroded by lazurite, and some lazurite crystals formed as a result of recrystallization of an initial sodalite-group mineral. Hypothetically, anionic and neutral extra-framework components (Cl − , OH − , F − , AlF ratio may be an indication of oxygen and sulfur dioxide fugacities during mineral formation [23].…”

“…The ESR spectrum of the ZK sample contains one broad band with a g-factor of 2.030. This signal is also associated with the S3 •centers [8,47]. It is noted [46] that the presence of a single isotropic band in the ESR spectrum of ultramarine samples with a high sulfur content is associated with the presence of the S3 •radical anions having the same local environment in sodalite cages.…”

“…•− centers [8,47]. It is noted [46] that the presence of a single isotropic band in the ESR spectrum of ultramarine samples with a high sulfur content is associated with the presence of the S 3…”

“…The samples KL and ZK are characterized by different n values, 0.164 and 0.147, respectively. The powder X-ray diffraction pattern of the ZK sample has been discussed in detail elsewhere [8]. Powder data for the samples MD and KL are given in Tables 3 and 4.…”

“…Raman spectroscopy is a very sensitive tool, which makes it possible to detect very low amounts of S 3 •− . Additional data on the S-bearing extra-framework groups in these minerals can be obtained by means of ESR, UV-Visible, and XANES spectroscopy [8,23,24,30,31]. In particular, radical anions S 2 •− and S 3 •− can be detected by means of ESR, absorption spectroscopy in the UV-Vis range, and luminescence spectroscopy.…”