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

摘要

Coherent intergrowths, at the lattice scale, between cuprobismutite (N = 2) and structurally related paderaite along both major axes (ISA and 17 A repeats) of the two minerals are reported within skarn from Ocna de Fier, Romania. The structural subunit, DTD, 3 layers of paderaite, is involved at interfaces of the two minerals along the 15 A repeat, as well as in transposition of 1 paderaite unit to 2 cuprobismutite units along the 17 A repeat in slip defects. Lattice images obtained by HRTEM across intervals of 200--400 nm show short- to long-range stacking sequences of cuprobismutite and paderaite ribbons. Such nanoscale slabs mimic jam-scale intergrowths observed in back-scattered electron images at three orders of magnitude greater. These slabs are compositionally equivalent to intermediaries in the cuprobismutite-paderaite range encountered during microanalysis. Hodrushite (A' = 1.5) is identified in the Jim-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 ISA, 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 paderaite. 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 paderaite 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 jim-scale implies coupling between diffusion-controlled structural modulation. and rhythmic precipitation at the skarn front during crystallization.