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American Mineralogist; April 2008; v. 93; no. 4; p. 632-643; DOI: 10.2138/am.2008.2700
© 2008 Mineralogical Society of America
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Thermal behavior of a Ti-rich phlogopite from Mt. Vulture (Potenza, Italy): An in situ X-ray single-crystal diffraction study

Gennaro Ventruti1, Michele Zema2,3, Fernando Scordari1,* and Giuseppe Pedrazzi4

1 Dipartimento Geomineralogico, Università degli Studi di Bari, Via E. Orabona 4, I-70125 Bari, Italy
2 Dipartimento di Scienze della Terra, Università degli Studi di Pavia, via Ferrata 1, I-27100 Pavia, Italy
3 CNR-IGG, Sezione di Pavia, via Ferrata 1, I-27100 Pavia, Italy
4 Dipartimento di Sanità Pubblica, Sezione di Fisica, Plesso Biotecnologico Integrato, via Volturno 39, I-43100 Parma, Italy

Correspondence: * E-mail: f.scordari{at}geomin.uniba.it

The structural evolution of a trioctahedral mica from Cava St. Antonio, Mt. Vulture, Potenza, Italy, has been studied in the temperature range 100–1023 K using in situ single-crystal X-ray diffraction techniques. The sample used shows a Ti-rich composition close to the phlogopite-annite join with the following crystal-chemical formula: (K0.85Na0.11Ba0.03)(Al0.17Mg1.98Mn0.01Fe0.292+Fe0.373+Ti0.18)(Si2.75Al1.25) O10.66(F0.13OH1.20Cl0.01). In the present study, the chemical and structural changes and the deprotonation process involved during heating have been characterized. Analysis of the data showed that a, b, and c cell parameters expand almost linearly up to 823 K, while the β angle remains constant. A sharp decrease in the unit-cell dimensions was observed at 873 and 1023 K. Linear trends obtained during reversal experiments from 1023 K to room temperature demonstrated the irreversibility of these events.

Structure refinements of single-crystal XRD data collected at 100, 200, 298, 473, 673, 873, and 1023 K converged to 2.14 ≤ R (%)≤ 8.47, 2.47 ≤ Rw (%) ≤ 10.83. In the temperature range 100 673 K, the thermal expansion along the c direction is mainly due to interlayer thickness dilation. The tetrahedral ring approaches the ideal hexagonal shape with increasing temperature to match the expanding octahedral sheet. In the range 873–1023 K, a strong shrinking of the interlayer is associated with the shortening of the M1-O4 and M2-O4 distances and to the consequent reduction of octahedral thickness. Such structural features indicate the occurrence of Fe oxidation process, involving loss of structural H, which is responsible for a phase transition. Mössbauer spectroscopy supported this hypothesis.

Key Words: Trioctahedral mica • single-crystal X-ray diffraction • deprotonation • thermal expansion






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