Iron-Rich Saponite (Ferrous and Ferric Forms)

Abstract

Clayey fragments colored deep bluish-green are widely found in glassy rhyolitic tufts at Oya, Tochigi Prefecture. In room-air the color changes to black or gray within one hour and finally to brown in a few weeks. The fragments are composed of an intimate mixture of two kinds of smectite: a ferrous iron-rich smectite (IR) with bo : 9.300 ~; and an iron-poor smectite(lP) with bo = 9.030 A. Microscopic examination shows a vesicular texture and that IR occurs at the core and IP at the mar- ginal parts of each vesicle. Analysis by EPMA gave the following structural formulas: IR, (Na0,60- Ko.04Ca0.44) (Mg~.04Fe 2+ A1o02) (Sir.36All.64)O20(OH)4; IP, (Na0.~2K0.08Ca0.26) (Mgog0Fe2+.A1254) 3"98 " ' O'9a ' (SiT.66A1o.~4)O20(OH)4. IR has a much larger amount of iron in trioctahedral sites than that found in any earlier data. Acid-dissolution data, infrared absorption spectra, Eh-values, and DTA and TG curves are also given. Ferrous iron in the structure is easily oxidized in room air with loss of protons from the clay hydroxyls and with contraction of the lattice. We call the IR before and after oxidation the ferrous and ferric forms, respectively, of iron-rich saponite. They strongly suggest the existence of the iron- analogue of saponite. On exposed weathered surfaces in the field, brown fragments tend to be differ- entiated into two parts: one light yellow montmorillonite-beidellite; the other a brown incrustation due to hisingerite.