Micron- to nano-scale intergrowths among members of the cuprobismutite series and paderaite: HRTEM and microanalytical evidence

Copyright © 2004 The Mineralogical Society

Coherent intergrowths, at the lattice scale, between cuprobismutite (N = 2) and structurally related padraite along both major axes (15 Åand 17 Årepeats) of the two minerals are reported within skarn from Ocna de Fier, Romania. The structural subunit, DTD, 3 layers of padraite, is involved at interfaces of the two minerals along the 15 Årepeat, as well as in transposition of 1 padraite unit to 2 cuprobismutite units along the 17 Årepeat in slip defects. Lattice images obtained by HRTEM across intervals of 200 -400 nm show short- to long-range stacking sequences of cuprobismutite and padraite ribbons. Such nanoscale slabs mimic µm-scale intergrowths observed in back-scattered electron images at three orders of magnitude greater. These slabs are compositionally equivalent to intermediaries in the cuprobismutite-padraite range encountered during microanalysis. Hodrushite (N = 1.5) is identified in the µm-scale intergrowths, but its absence in the lattice images indicates that, in this case, formation of polysomes between structurally related phases is favoured instead of stacking disorder among cuprobismutite homologues. The tendency for short-range ordering and semi-periodic occurrence of polysomes suggests they are the result of an oscillatory chemical signal with periodicity varying from one to three repeats of 15 Å, rather than simple 'accidents' or irregular structural defects. Lead distribution along the polysomes is modelled as an output signal modulated by the periodicity of stacking sequences, with Pb carried within the D units of padraite. This type of modulator acts as a patterning operator activated by chemical waves with amplitudes that encompass the chemical difference between the minerals. Conversion of the padraite structural subunit DTD to the C unit of cuprobismutite, conserving interval width, emphasizes that polysomatic modularity also assists interference of chemical signals with opposite amplitudes. Observed coarsening of lattice-scale intergrowths up to the µm-scale implies coupling between diffusion-controlled structural modulation, and rhythmic precipitation at the skarn front during crystallization.

C.L. Ciobanu, A. Pring and N.J. Cook