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Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions

¹ Dipartimento di Geoscienze, Università di Padova, Via Giotto 1, Padova, 35137, Italy
² C.N.R., Istituto di Geoscienze e Georisorse, Sezione di Padova, Corso Garibaldi 37, Padova, 35137, Italy
³ Institute of Geosciences, Shizuoka University, Oya 836, Shizuoka, 422-8529, Japan
⁴ Dipartimento di Scienze della Terra, Università di Ferrara, Via Saragat 1, Ferrara, 44100, Italy
⁵ Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, Padova, 35131, Italy

Correspondence: ^* E-mail: bernardo.cesare{at}unipd.it

Precise chemical composition, including Fe³⁺ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mössbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T > 800–850 °C and P = 5 ± 1 kbar.

The full analyses were normalized on the basis of [O_12–(x+y+z)(OH)_xCl_yF_z]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H₂O-poor (OH = 0.86 pfu), has X_Mg = 0.49 and Fe³⁺/Fe_tot 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites.

In the host granulite, fine-grained biotite is Fe³⁺-free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and X_Mg in the ranges 0.26–0.36, 0.52–0.67, and 0.67–0.77, respectively. The number of octahedral vacancies is relatively large (0.10–0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of X_Mg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, X_Mg, and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant.

These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti⁴⁺ + 2O^2– = (Fe,Mg)²⁺ + 2OH^– and the "fluorination" Ti⁴⁺ + 2O^2– = (Fe,Mg)²⁺ + 2F^–. The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.

Key Words: Biotite • crystal-chemistry • granulite • pegmatite • retrograde diffusion • titanium