COSTO DE LA INSTALACIÓN CENTRALIZADA

5.1 Introduction.

The structure of anorthite CaAljSijOg was determined by Kempster et aL (1962), and subsequent refinements of the pure anorthite structure were carried out by Wainwright and Starkey (1971), Czank (1973), Kalus (1978), Wruck (1986) and Angel et al.

(1990), amongst others. The room temperature structure of anorthite belongs to the PI space group and has the approximate dimensions of 8 x 13 x 14 A. The cell consists of four subcells, each with the formula unit CaT^O, related by a centre of symmetry. When fully ordered. Si and A1 tetrahedra alternate so that each O atom has one Si neighbour and one A1 neighbour, in accordance with Loewenstein’s rule (Loewenstein, 1954). In reciprocal space, anorthite has four times the number of lattice points of albite. This gives rise to four types of reflections, these are classified as follows (Bown and Gay, 1958):

a) (hkl) with 1 even and (h+k)=even b) (hkl) with l=odd and (h+k)=odd c) (hkl) with l=odd and (h+k)=even d) (hkl) with l=even and (h+k)=odd

Analysis of the PI, II and C l plagioclase structures (Wenk and Kroll, 1984) revealed the structural contribution to 6, c and d type superstructure reflections using Fourier analysis.

In the basic albite type structure with space group C l, all four tetrahedra ring units are identical within a chain. In the II structure two rings are no longer equivalent and there is reverse Al,Si distribution (Loewenstein, 1954). This causes the z-axis to double and leads to the occurrence of b superstructure reflections in addition to the "basic" «-reflections of albite. When symmetry is further reduced from II to PI the two parallel chains at x,y~0 and x,y~l/2 are no longer equivalent and gives rise to extra superstructure reflections c and d. The main contribution for c and d reflections comes from Ca occupancies. The c-reflections are also linked to Og, Oq, 0^(1) and Tj atoms. The d-reflections are linked to the T^, TjCm) and Og(m) atoms.

Well ordered end-member anorthite has symmetry PI at room temperature and pressure. On heating to temperatures above 240*C anorthite undergoes a phase transition to a structure with 11 symmetry, as evidenced by the disappearance from the diffraction pattern of the superstructure reflections c and d (Brown et al., 1963, and studies thereafter). Application of pressure to the anorthite structure at room temperature (Angel et aL, 1988; Angel, 1988) also results in a phase transition to a structure with 11 symmetry, the same as observed at high temperatures. The pressure at which this transition occurs has been shown to increase with albite content of the anorthite (Angel et aL, 1989) and with the degree of disorder of the A1 and Si within the tetrahedral framework of the structure (Angel, 1992). The transition is non- quenchable on the time scale of the X-ray experiments, and neither the high pressure or high temperature phases can be recovered at ambient conditions. The structure of the high pressure phase, in contrast to that at high temperatures, shows single sites for the Ca atoms within a framework with true 11 symmetry.

1 have undertaken a series of in-situ measurements of the cell parameters of end member anorthite at simultaneous high pressures and temperatures in order to determine by in-situ reversals, whether the phase transitions at high pressure and high temperature are related.

5.2 The High Temperature P l-Il Phase Transition in Anorthite.

The reversible P l-Il transition was discovered by Brown et al. (1963), who reported the transition temperature to be 350*C. Using precession photography at high temperatures they demonstrated that the c reflections (h+k=even, l=odd) gradually became diffuse with increasing temperature and could not be detected beyond the critical temperature, T^. Bruno and Gazzoni (1967) using film techniques at high temperatures, observed a continuous and reversible change in the diffuseness of c-

reflections. The reflections were seen to disappear between 125*C and 350*C depending upon the composition and thermal history of the sample. Using a single crystal sample, Foit and Peacor (1967) reported the diffuseness increased between 25"C and 350’C. However, the authors were ambiguous as to whether the c-reflections were absent above or merely too weak and diffuse to be observed. Frey et al.

(1977) made neutron diffraction measurements of the intensities of some c-reflections in the elastic setting, which go to zero at a critical temperature of 237'C.

Laves et al. (1970) were the first to suggest that c-reflections persist beyond T^. These single crystal studies reported that the a and b reflections remain sharp until the melting point at 1540*C, while c-reflections become diffuse with increasing temperature but remain observable up to and above 400’C. X-ray studies carried out by Czank (1973) reported that although the intensity of c-reflections becomes very low beyond T^, they do not disappear entirely. Adlhart et al. (1980) made neutron diffraction measurements of the intensities in an elastic setting. They found that although the intensities of some c-reflections go to zero at 7^, for others continually decreasing intensities remain above (241'C). Electron diffraction and microscopic studies of the high temperature transition are presented in the studies by Chose et al.

(1988), Hatch and Chose (1989) and Van Tendeloo et al. (1989). These papers argued that transitions such as these, are associated with the formation of antiphase domains (APDs). The interfaces separating these domains are observable by diffraction contrast

in an electron microscope. At temperatures just below the APD boundaries (APBs) become unstable and begin to change configuration. At the same time small APDs in the form of small closed loops form and disappear with slight changes in temperature (1 or 2*C). At temperatures of approximately 150*C above T^, the frequency of the APB fluctuations increases to such an extent that individual interfaces can not be seen and a shimmering effect is observed on imaging in dark field images using diffuse c-

reflections. On cooling through the APBs reappeared. Using this method the critical temperature was placed at 243*C.

These differing observations at high temperature led to the formulation of a number of models to explain the behaviour of anorthite during the transition: