Crystal chemical variations in Ba-rich biotites from gabbroic rocks of lower crust (Ivrea Zone, NW Italy)

Abstract

Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.28<Fe/(Mg+Fe)<0.66]. Biotites from gabbros are enriched in BaO and TiO2 (up to 7.14 and 9.32 wt%, respectively) with respect to those of the diorites (up to 1.26 and 6.26 wt%, respectively). Systematic compositional variations support the substitution model 2(IV)Si + ((VI)R2+) half arrow left over half arrow right 2(IV)Al + (VI)Ti (R2+ = Fe + Mg + Mn) in gabbros and (IV)Si + (VI)Al half arrow right over half arrow left (IV)Al + (VI)Ti in diorites. A predominance of disordered stacking sequences, coexisting with 1M, 2M1 and 3T polytypes was observed in all biotites. It was possible to carry out structural refinements only on three biotites-2M1 from diorites (R-values between 2.68 and 3.77) 'and one biotite-1M from gabbros (R-value=3.09). It was shown that: (1) the reduced thickness of the tetrahedral sheet in Ba-rich biotites supports the coupled substitution (IV)Si + (XII)K half arrow left over half arrow right (IV)Al + (XII)Ba; (2) the interlayer site geometry is affected by the whole layer chemistry and does not reflect only local chemical variations; (3) in two samples of the 2M1 polytype, the M(1) octahedral site is larger and more distorted than the M(2) sites because of the preferential ordering of Fe2+ in the M(1) site, whereas one sample shows complete cation disorder in the octahedral sites. Biotite-1M shows that Fe2+ can also be located in the M(2) site. Some of the differences between the biotites of gabbros and diorites (e.g. Ba concentration and exchange vectors) may be linked to the host rock composition and to its crystallization process. Biotite occurs in trace amounts in gabbros and its crystallization is related to the interstitial melt which contributed to the adcumulus growth of the main rock forming phases and became highly enriched in K, Ba and Ti. Diorites are the result of equilibrium crystallization of a residual melt rich in incompatible elements, where biotite is a major constituent

Solid‐state nuclear magnetic resonance investigation of synthetic phlogopite and lepidolite samples

“…[25] The unit cell is outlined correlation between the chemical composition and the appearance of the 2M 1 polytype is known. [28] 3.1.1 | 27 Al and 17…”

Chemical control of 3T stacking order in a Li-poor biotite mica

“…Thus, compositional control over long-range-ordered 3T stacking in trioctahedral micas appears to emerge at least for Li-rich compositions, but this stacking trend is not common in rock-forming, Li-poor biotite. In addition to the study by Baronnet et al (1993), 3T sequences with limited stacking order have been reported by TEM investigations in a H-poor, Fe-rich phlogopite from the Valle del Cervo (Italy) plutonic body (Bigi et al 1993;Bigi and Brigatti 1994), and in the H-poor and Fe 3+ -rich oxybiotites from the volcanic Ruiz Peak rhyodacites, New Mexico (Kogure and Nespolo 2001).…”

“…Although some authors maintained that chemical composi-* E-mail: r.fregola@geomin.uniba.it tion is not a likely factor controlling mica polytypism (Takeda and Ross 1975;Bigi et al 1993;Bigi and Brigatti 1994), others considered compositional variation itself a structural factor, thus playing a dominant role in controlling the formation of different mica polytypes (Smith and Yoder 1956;güven 1971). In addition, there are high-resolution and analytical transmission electron microscopy (HRTEM and ATEM, respectively) studies showing chemical differences among different coexisting stacking sequences (Baronnet and Kang 1989;Baronnet et al 1993;Xu and Veblen 1995).…”

Ba- and Ti-enriched dark mica from the UHP metasediments of the Seve Nappe Complex, Swedish Caledonides

“…Barium-rich micas are known from magmatic rocks (e.g. Bigi et al 1993;Edgar 1992;Gaspar, Wyllie 1982;Kogarko et al 2005;Mansker et al 1979;Thompson 1977;Wendlandt 1977;Zhang et al 1993), metamorphosed ore deposits (Chabu, Baulège 1992;Dasgupta et al 1989;Frimmel et al 1995;Frondel, Ito 1967;Gnos, Armbruster 2000;Holstam, Larsson 2000;Matsubara et al 1976;Yoshii et al 1973a, Tracy 1991 and metamorphic rocks (e.g. Bol et al 1989;Bucher-Nurminen 1982;Glassley 1975;Harlow 1995;Kretz 1980;Pattiaratchi et al 1967;Rice 1977;Solie, Su 1987;Tracy, Beard 2003).…”