Real Structure of Micas and Amphiboles from the Archean Complex of the Kola Superdeep Borehole (KSDB)

Nikita Ovchinnikov (nikita@ad.iggp.ras.spb.ru)¹, Miriam Babushkina (msb@mb2171.spb.edu)¹, Larisa Nikitina (nikita@ad.iggp.ras.spb.ru)¹, Antonina Yakovleva², Olga Chernova³ & Alisa Yugina

¹ Institute of Precambrian Geology and Geochronology, Russian Academy of Science, Makarova emb. 2, St-Petersburg, 199034, Russia

² Scientific Industrial Centre Kola Superdeep, Zapolyarny, Murmansk region, 184415, Russia

³ St.-Petersburg StateUniversity(Mineralogy department), St-Petersburg, Universitetshaya emb. 7/9, 199034, Russia

The Kola superdeep borehole (12261 m) is a unique object to investigate the composition, crystal structure and physical properties of rocks, minerals and fluides, which are in equilibrium with physical-chemical and thermodynamical conditions of the Middle crust. More than 30 actinolites, cummingtonites and thrioctahedral micas from Archean (depth interval 7896-11334 m.) and the Proterozoic complex amphibolites (6451 and 6433 m.) were studied. The homogeneity of all samples was checked up by optical and powder X-ray diffraction methods. Amount of impurities of other minerals are less than 3%. The chemical compositions of minerals were determined by "wet" chemistry and electron microprobe analysis. The degree of oxidation state of iron, the cation order-disorder degree, the formes of water, the quantity of vacancies in minerals structure were studied by Mössbauer and Infrared methods. The discrepancy between Mössbauer, "wet" chemistry data and crystal chemistry calculations of microprobe data concerning the iron oxidation state in minerals was established. The part of ferric iron due to Mössbauer data is 1.5-2 times lower. More vacancies in octahedral layer and molecular water in the structure of phlogopites from KSDB in comparison with phlogopites of metamorphic and igneous rocks on the surface were established. Probably these vacancies may play a role of traps for volatiles. At the same time in the structure of actinolites and cummingtonites the OH-groups and molecular water were not established. The cation deficit in octahedral sites in actinolites structure was not determined also. The dependence of iron and magnesium order degree on depth for phlogopites were established. The specific features of crystal structure of KSDB micas may reflect the influence of conditions of the Middle crust on the properties of crystalline substances. This work is supported by RFFI 99-05-65293.

High Pressure Twisting of Tetrahedra and Amorphization in (alpha)-quartz

Nikolay Ovsyuk (ovsyuk@uiggm.nsc.ru) & Sergei Goryainov (svg@uiggm.nsc.ru)

Institute of Mineralogy and Petrology, Novosibirsk, 630090, Russia

Recently, a large number of computer simulations of the (alpha)-quartz structure at high pressures have been made. Significant distortions of SiO₄ tetrahedra should be taken into account in these simulations when the relations between various microscopic parameters become numerous and complex. In this case, it is difficult to understand what sort of structure transformations are responsible for the dominant mechanisms of tetrahedral deformation. In order to clarify these mechanisms, we propose a simple valence force model, in which only two parameters, most sensitive to pressure, have been distinguished. The tilt angle of SiO₄ tetrahedra is one of two parameters, which was detected previously. It is commonly used to describe (alpha)-ß phase transition of quartz. This angle is the main parameter of structure deformation at high pressure, and it is assumed to lead to instability, resulting in amorphization. We have fond the second parameter, carefully analyzing numerous parameters of tetrahedral distortions, mentioned in the literature. We have discovered that these parameters can be reduced to a single parameter, i.e., the twist angle of tetrahedra. Earlier this angle was not considered as the order parameter at high pressures. The twist angle of tetrahedra can be defined as deviation from a 90° angle between two opposite symmetrical tetrahedron edges in (alpha)-quartz. As a result, we have managed to reveal some peculiarities, unnoticed in numerical calculations, i.e., the tilt angle at high pressures tends to saturation; the twist angle, on the other hand, starts to change nonlinearly. Therefore this is the angle which is mainly responsible for structure instability. This information seems to be important; despite a considerable quantity of experimental observations and theoretical simulations of (alpha)-quartz amorphization at high pressure, there is no correct description of the microscopic origin of the softening of the phonon modes, which are responsible for the elastic instability. In conclusion it may be said that the valence force model of (alpha)-quartz deformation with good allowance for the tetrahedra distortion is proposed (Ovsyuk et al.,1999). According to this model, the tetrahedra tilt angle tends to be saturated at high pressures and, consequently, the effective stiffness of the structure, relative to that angle, increases. This fact evidences for the increase of the structure stability and cannot lead to instability. At the same time, the parameter, lesser in magnitude, i.e., the twist angle, starts to grow nonlinearly with pressure. This causes softening of the lowest acoustical branch and loss of the tetrahedra stability, relative to twisting, thus, causing transition into the triclinic phase with the subsequent structure amorphization.

Ovsyuk NN & Goryainov SV, Phys. Rev. B, 60, 14481-14484, (1999).

In situ Atomic Force Microscopy Investigation of Biotite Dissolution in Acid Conditions

Katavut Pachana (biotite59@hotmail.com), Giovanni DeGiudici (dgdg@ipgp.jussieu.fr) & Pierpaolo Zuddas (zuddas@ipgp.jussieu.fr)

Laboratoire de Geochimie des Eaux (CNRS 7052) Universite Paris 7 and IPGP. 2, place Jussieu, case 7052 F75251 Paris cedex 05, France

The reaction of biotite dissolution was investigated by experiments in the liquid cell of an Atomic Force Microscopy at room temperature and at a pH value of 1. The morphological evolution of a freshly cleaved (00l) surface and the chemical composition of the interacting solution introduced by peristaltic pump under high flow condition were analysed for 5 hours. Our AFM images in air (before interaction) show that the periodicity resolution at the molecular scale of the (00l) pristine surface, consists on basal oxygen groups with hexagonal-like arrangement interpreted to correspond to the biotite tetrahedral sheet. The reaction of dissolution produces etch pits with stair-step pattern having the first step of 0.5 (± 0.1) nm height. We estimated that the average velocity of the step retreat is 2.97 nm/min, while they deepening is 0.006 nm/min. The formation of pits produces an increase of the (hk0) lateral surface area evaluated to 3100 nm²/min (image size: 2250nm x 2250nm). The kinetic rate of biotite dissolution estimated by the variation of silica in solution normalised to the basal surface area was found to be constantly equal to 30 nmoles per square meter per second while the rate estimated by the variation of the magnesium plus iron in solution normalised to the lateral surface area was found to be more than one order of magnitude lower. We propose that the reaction of biotite dissolution in acid conditions proceeds via edge attach where steps correspond to the tetrahedral and octahedral or tetrahedral and interlayer sheets, and that the reactivity of the basal (001) is significantly lower than the lateral (hk0) surface.