ON THE SHAPE OF THE FIRST ILLITE X-RAY DIFFRACTION-REFLECTION, CRYSTALLINITY, AND INCIPIENT METAMORPHISM

In a profile along the Swiss Alps 88 lithologically different sediment ary rocks, formed under diagenetic, low- and high-anchimetamorphic con ditions, were sampled from 41 outcrops and prepared under normalized c onditions to obtain <2 mu fractions. The samples were analyzed chemica lly by non-destructive X-ray fluorescence analysis, and X-ray diffract ometry (air-dried, glycolated); from all specimens temperatures ex-flu id inclusions in authigenic quartz had been determined previously. The XRD evaluation comprised: a- half-width determination of the complex, unresolved 10 Angstrom basal reflection (Kubler width); b- deconvolut ion of this complex reflection using Pearson functions for the three r eflections of illite-muscovite, smectitic phase/smectite-illite mixed- layer, and chlorite in the angular range of 3-13 degrees 20 (CuK alpha radiation, automatic divergence slit, optimized counting statistics b y slow motion measurement); c- domain size determination after Warren- Averbach using a muscovite single crystal (002) reflection as a refere nce, The 10 Angstrom A complex reflection is formed by the first illit e basal reflection and in diagenetic/low-anchimetamorphic specimens by an additional mineral phase, expandable under glycol, and labelled he re 'smectitic phase'. This phase has not been idientified so far, but may be a mixed-layer mineral consisting of chlorite, illite,..,smectit e. Though there exists for certain specimens a clear correlation betwe en temperature ex-fluid inclusions and a the overall half-width (Kuble r width), but also b- the half-width, the curve exponents(curve form=G aussor Cauchy-like) and the peak position s of the deconvoluted illite and smectitic phase, this correlation is neither in all diagenetic, n or in all anchimetamorphic specimens preponderant. Domain sizes of coe xisting air-dried illites and smectitic phases are, however, significa ntly correlated (r=0.93, N=88), as are the half-widths and d- spacings from the smectitic phase (r=0.90, N=88), on which both temperatures d epend, at least trendwise. Several reasons are responsible for the som etimes weak interdependence of peak shape and illite crystallinity/inc ipient metamorphism: the general shape of the clay 10A complex reflect ion is influenced by the height, position, half-width, symmetry and fo rm of its contributors (e.g., illite-muscovite, smectitic phase, chlor ite) on the one hand, and by instrumental, preparational, and specimen -related factors on the other, As a consequence, neither the overall h alf-width (Kubler width), nor the deconvoluted illite half-width is ne cessarily linked with illite crystallinity and incipient metamorphism alone. Notably, the grinding impact on sheet silicates during sample d ressing may influence the symmetry of their basal reflections and half -widths, as can be demonstrated experimentally. But the chemical compo sition of the specimen may contribute also, for specimens from one and the same outcrop display, in some cases, different half-widths of ill ite and smectitic phase together with a strong chemical difference of the <2 mu fractions involved.