Home – Home – Jornals – Russian Geology and Geophysics 2020 number 1

Hydrotalcite supergroup minerals stichtite, pyroaurite, iowaite, and woodallite form a complex solid-solution system at the Kyzyl-Uyuk locality (Terekta Ridge, Gorny Altai, Russia). The diversity of these minerals is due to: (1) subdivision by anionic interlayer composition into carbonate (stichtite and pyroaurite) and chloride (iowaite and woodallite) species and (2) isomorphism of M ³⁺ cations, mainly between Cr- (stichtite and woodallite) and Fe³⁺-dominant species (pyroaurite and iowaite), with a quantitative predominance of stichtite and iowaite. Most of the studied samples correspond to stichtite and woodallite with high Fe³⁺ contents or pyroaurite and iowaite with high Cr³⁺ contents. According to vibrational (IR and Raman) spectroscopy data, the interlayer Cl^- is partially substituted by OH^- rather than CO₃^2- groups. We suppose that the presence/absence of a band in the region 1400-1350 cm^-1 in the Raman spectra of stichtite can be explained by the local distortion of triangular CO₃ groups. Stichtite and iowaite occur here in polytypic modifications 3 R and 2 H that are the most widespread for the hydrotalcite supergroup minerals. For both minerals, the polytype 3 R strongly dominates over 2 H ; the lowest 3 R /2 H ratio determined for the Terekta iowaite and strichtitle is 2:1. The Altai stichtite is close in 3 R /2 H to the stichtite from Tasmania (Australia) and differs significantly from that in Transvaal samples (South Africa).