77 d.C. â Pliny the Elder: transparent mineral with high splendor. 1540 â P. Plateanus ... 1850 â J.D. Dana: the Moscovia «white» mica is monoclinic and has a large axial angle ... talc + phengite and two new phengite barometers. European ...
WHITE MICAS: INDICATORS OF P, T, X CONDITIONS IN THE METAMORPHIC REALM Annibale Mottana
Petrological «white» micas ≈ Francesco Paolo Sassi
How did «white» micas enter Mineralogy? 77 d.C. – Pliny the Elder: transparent mineral with high splendor 1540 – P. Plateanus : mica IT (in editing G. Agricola’s «Bermannus») 1728 – J. Woodward : mica EN (substituting for Glimmer or Splendor) 1801 – R.J. Haüy: the Vesuvius «brown» mica is orthorhombic (renamed «biotite» by J.F.L. Hausmann [1847] after J.-B. Biot, who had determined [1816] it to be pseudo-uniaxial) and has a large Fe-Mg content 1841 – A. Breithaupt: «white» mica has an axial angle varying from small to large and is high in potassium 1850 – J.D. Dana: the Moscovia «white» mica is monoclinic and has a large axial angle 1878 – G. Tschermak: a phengite is a «white» mica rich in Si
Present I.M.A. classification of the micas (Rieder et al., 1998) 1) «White» micas are never mentioned throughout the text. 3) All micas previously mentioned as «white» or «light» micas are included in the dioctahedral group, comprising 11 species. 2) Phengite was elevated to a series name for solid solutions involving muscovite, aluminoceladonite, and celadonite.
Other micas traditionally included among the «white» ones
Macro-, micro- and atomic aspects of phengite
The compositional space of dioctahedral micas and the Si (Mg+Fe) apfu ratio of phengites l.s. & s.s.
Tischendorf et al, variis locibus
Chemical dependence of Si vs. (Mg+Fe), in contrast with the irregularity of the Fe redox ratio
Baldelli et al. Lithos 1989
The dependence of the interlayer occupancy upon the octahedral layer occupancy
Baldelli et al. Lithos 1989
Coexisting phengite and paragonite in the «calcschists» of the Voltri Group
paragonite
phengite
Liborio et al. 1970
Coexisting sheet silicates: phengite & talc Massonne HJ, Schreyer W (1989) Stability field of the high-pressure assemblage talc + phengite and two new phengite barometers. European Journal of Mineralogy 1: 391-410.
Interplay of compositional and structural problems in real phengites • Several chemical substitutions are involved in the structural and chargeequilibration processes by which the tetrahedral (T) and octahedral (M) sheets of phengites change their sizes to match structurally. • Theoretically, the two substitutions Si (Mg+Fe) should balance; in particular, they are strictly obliged to maintain a ratio that accounts for the dioctahedral structure (M = 2), which is emblematic of all white micas. However, many phengites do not equilibrate compositionally, either for the excess charge in the T sheet induced by a Si > 3 apfu value greater than the charge defect determined in the M sheet even after reducing some Fe3+ to Fe2+, or because there is still an excess of total M cations after such a reduction. • This is the basic reason behind the suggestion that there is indeed a partial occupancy of the third octahedral site possible in micas (2 ≤ M ≤ 3) i.e., the site that is vacant in ideal dioctahedral micas. This may indeed occur, at least computationally, however to a very limited extent (
