Yoderite, a New Hydrous Magnesium Iron Aluminosilicate from Mautia Hill, Tanganyika

Duncan McKie and A. J. Radford
Dept. of Mineralogy and Petrology, Downing Place, Cambridge
Geological Survey, Causeway, Salisbury, Southern Rhodesia

Summary: Yoderite occurs as a major constituent in a quartz-kyanite-tale schist, and is a high-pressure phase formed by the reaction Ky + Tc → Yd + Qu. The monoclinic unit-cell, a 8·10 Å., b 5·78 Å., c 7·28 Å., β 106°, space-group P21 or P21m, contains approximately (Mg2·0Ca0·2Fe0·5Al5·3Si4·0O17·6(OH)2·4. The structure appears to be related to that of kyanite, with which the mineral is intergrown; it is not an analogue of staurolite. The three strongest powder lines are 3·50 Å. vvs, 3·03 Å. vs, 2·61 Å. s. Specific gravity 3·39. Subsidiary reflections, similar to those of the intermediate plagioclases, have indices with non-simple fractional values of k. Optical properties are α 1·689 pale Prussian blue, β 1·691 indigo, γ 1·715 light olivegreen, absorption β > α > γ, 2Vγ25°, optic axial plane {010}, γ:[001] ≈ 7° in the obtuse angle β. An interpretation of the colour on the electron-exchange hypothesis Fe2+ ⇌ Fe3+ + e, is suggested. A kinetic study has been made of the disappearance of the subsidiary reflections in the range 700°C. to 820°C.; further heating to 850°C. produces the metastable appearance of mullite, and at temperatures above 1100°C. the equilibrium anhydrous assemblage indialite + sapphirine appears.

Mineralogical Magazine; December 1959 v. 32; no. 247; p. 282-307; DOI: 10.1180/minmag.1959.032.247.02
© 1959, The Mineralogical Society
Mineralogical Society (www.minersoc.org)