Hans P. Eugster

Hydrous Sodium Silicates from Lake Magadi, Kenya: Precursors of Bedded Chert

Two new hydrous sodium silicates, NaSi7O13(OH)3�3H2O (magadiite) and NaSi11O20.5(OH)4�3H2O (kenyaite), were found in lake beds at Lake Magadi, Kenya. Both are well-crystallized layered silicates with large basal spacings. Concretions within the magadiite bed consist of kenyaite or quartz (chert) in the center, surrounded by kenyaite. In dilute acids magadiite and kenyaite are converted to 6SiO2�H2O (SH), the first known crystalline hydrate of silica. The magadiite bed probably represents a chemical precipitate from alkaline brines. Percolating waters convert magadiite to kenyaite and eventually to chert. Thus a mechanism has been outlined for the formation of bedded chert deposits through inorganic precipitation. Alternations between silica-rich and iron-rich bands of iron formations may be due to concentration cycles in alkaline lakes.

The system Fe-Si-O: Oxygen buffer calibrations to 1,500K

AbstractThe five solid-phase oxygen buffers of the system Fe-Si-O, iron-wuestite (IW), wuestite-magnetite (WM), magnetite-hematite (MH), quartz-iron-fayalite (QIF) and fayalite-magnetite-quartz (FMQ) have been recalibrated at 1 atm pressure and temperatures from 800°–1,300° C, using a thermogravimetric gas mixing furnace. The oxygen fugacity,

Inorganic bedded cherts from the Magadi area, Kenya

f_{{\text{O}}_{\text{2}} }

Mineral equilibria in the six-component seawater system, Na-K-Mg-Ca-SO4-Cl-H2O at 25°C. II: Compositions of the saturated solutions

was measured with a CaO-doped ZrO2 electrode. Measurements were made also for wuestite solid solutions in order to determine the redox behavior of wuestites with O/Fe ratios varying from 1.05 to 1.17. For FMQ, additional determinations were carried out at 1 kb over a temperature range of 600° to 800° C, using a modified Shaw membrane. Results agree reasonably well with published data and extrapolations.The reaction parameters K, ΔGro, ΔHro, and ΔSrowere calculated from the following log

Depositional Environment of the Green River Formation of Wyoming: A Preliminary Report

f_{{\text{O}}_{\text{2}} }

Mineral equilibria in a six-component seawater system, Na-K-Mg-Ca-SO4-Cl-H2O, at 25°C

/T relations (T in K):