The ultrastructure of pyroxenoid chain silicates. I. Variation of the chain configuration in rhodonite

Abstract

Two samples of the silicate mineral rhondonite have been examined, in the natural state and after heat treatment, by high-resolution electron microscopy. Planar faults parallel to (001) and (ll0) have been observed. Computer-simulated images and optical diffraction studies have confirmed that at (001)faults strips of pyroxmangite structure are inserted into the parent rhodonite matrix. The second type of fault has been found to be equivalent to the stacking faults observed in wollastonite, and similarities and differences between this type of fault in the rhodonite and wollastonite structures are discussed.

The ultrastructure of pyroxenoid chain silicates. III. Intersecting defects in a synthetic iron–manganese pyroxenoid

“…The presence of disordered intergrowths in various types of pyroxenoid chain silicates has been firmly established by high-resolution electron microscopy (Alario-Franco, Czank & Liebau, 1980;Ried & Korekawa, 1980) The original assignment of defects observed in (GOD lattice images to variations in the chain periodicity has been confirmed by combined computer simulation/electronmicroscopic studies (Jefferson, Pugh, Alario-Franco, Mallinson, Millward & Thomas, 1980), and it is now possible to observe the tetrahedral arrangement in such structures directly (Smith, Jefferson & Mallinson, 1981), hence assigning chain repeats unambiguously. The existence of defects of the types reported confirms the original suggestion (Liebau, 1972) that the pyroxenoids form a continuous structural series, adapting to variations in cation size/ pressure/temperature in a continuous manner.…”

The ultrastructure of pyroxenoid chain silicates. II. Direct structure imaging of the minerals rhodonite and wollastonite

“…Appropriate crystal projections, namely those corresponding to overlap of the metal cation positions with the tetrahedral sites, were indicated by image simulations, and experimental images were also obtained at 500 kV which matched closely with those simulated for very thin crystal regions. However, it is of considerable concern, given our long-term objectives of imaging, and interpreting, isolated structural defects at the atomic level in the pyroxenoids, as outlined here and elsewhere (Thomas, Jefferson, Mallinson, Smith & Crawford, 1979;Jefferson et al, 1980), that both the simulated and experimental images displayed such rapid variations in contrast with increasing crystal thickness, and that agreement between computation and experiment also appeared to worsen rapidly with thickness. Admittedly, more extensive calculations could be undertaken, in particular to ensure that sufficient diffracted beams were included to obtain a closer fit with the higher-resolution detail, especially at greater thicknesses.…”

“…Since variations within pyroxenoids often involve an alteration in the repeat length of the silicate chain, corresponding variations in fringe periodicities can usually be observed in simple lattice images. With these images, it has proved possible to investigate such intergrowths in rhodonite (Jefferson et al, 1980), where these are of a relatively simple type. Nevertheless, where intergrowths are of complicated form, such as those involving alterations from the chain-silicate pyroxenoids to the ring-type metasilicates, higher resolution is required since it is necessary to image individual atomic sites for unambiguous image interpretation.…”

Rhodonite-Pyroxmangite from Tanatz Alp, Switzerland

“…The two minerals can generally be distinguished on the basis of their chemical composition, as rhodonite contains a higher amount of Ca (more than 0.05 wt.%) than pyroxmangite. Rhodonite and pyroxmangite often occur together as a bladed intergrowth, in various types of ore deposits and Mn-rich lithologies (Ohashi et al, 1975;Jefferson et al, 1980;Pinckney and Burnham, 1988;Millsteed et al, 2005;Michailidis and Sofianska, 2010).…”