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A biotite crystal from D?epa (Serbia) was studied by means of X-ray diffraction topography-XRDT, high resolution and analytical transmission electron microscopy-HRTEM and ATEM, electron probe microanalysis-EPMA, and secondary-ion mass spectrometry-SIMS. The combination of EPMA data and SIMS results (H, Li, F, Cl) showed that the specimen is a Ti-rich magnesian annite. Its chemical composition, averaged on 15 spots, is as follows:
(K0.87Na0.05Ca0.01)(Fe2+1.36Mg1.25Ti0.22Al0.14Mn0.03Li0.01)(Si2.84Al1.16)O10[(OH)1.535O0.35F0.095Cl0.02]
The specimen is thus characterized by a very low Li content, and by a deficiency in (OH)+F+Cl (Σ = 1.65) and enrichment in O (Σ = 10.35), due to Ti-oxy substitution VIM2+ + 2(OH)- --> Ti4+ + 2O2- + H2.
The complementary use of XRDT and TEM techniques resulted to be a quite suitable methodological approach for this study. In fact, XRDT showed thin band diffraction contrasts in some defect regions, owing to either different coexisting polytypes or twins in contact through (001). Topographs were used as guide-map for sampling TEM specimens from these defect regions.
Selected area electron diffraction (SAED) patterns and HRTEM images showed that all the stacking sequences belonged only to subfamily-A of mica polytypes, characterized by (2nx60°) successive layer rotations. Moreover, HRTEM images along [-110] allowed the identification of three distinct types of stacking arrangements:
1.Dominant areas with 1Mr-n(120°) rotational disorder. Within these areas, a few repetitions of the five-layers as well as of the eight-layers inhomogeneous stacking arrangements, belonging to the 3T structural series, were also found.
2.An area, denoted as highly faulted 2M1, where variably long segments of the stacking sequence 2-2, identifying the 2M1 polytype, are broken-up by faults -22. The latter are frequent and sometimes periodic. Locally, the stacking faults are arranged in such a way that long-period inhomogeneous polytypic sequences, based on the 2M1 structural series, are identified.
3.A perfectly ordered 3T lamella, with the typical [222] stacking sequence that repeats itself over wide distances (some micrometers).
ATEM analyses highlighted that the chemical composition of 3T differs from that of the host matrix. In particular, it is markedly enriched in interlayer cations (K and especially Na), slightly enriched in Al, slightly depleted in Si, Mg and Fe.
Among the three homogeneous mica polytypes (1M, 2M1, 3T) belonging to subfamily-A, 3T is rather rare. It is mainly found in dioctahedral micas muscovite and phengite, and in Li-rich trioctahedral micas polylithionite and lithian siderophyllite. On the contrary, the occurrence of 3T stacking among Li-poor trioctahedral micas is quite rare, and usually on short distances at TEM scale. Thus, our finding of chemically differentiated 3T stacking in a Li-poor trioctahedral mica is an outstanding result. A possible chemical control on the stability of 3T stacking is suggested.
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