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Antigorite

GeoForschungsZenturm Potstdam, Division 4.1 Telegrafenberg, D-14473 Potsdam, Germany

View larger version (10K): [in this window] [in a new window]   Fig. 1. Selected phases of the system MgO-SiO2-H2O (MSH) and enlarged projection of the tie-line brucite - talc with the range of chemical variations of natural antigorites according to Mellini et al. (1987), defined by m-values (see text).

View larger version (36K): [in this window] [in a new window]   Fig. 2. Crystal structure of antigorite with m = 17. Structural data for this drawing are taken from Uehara (1998).

View larger version (146K): [in this window] [in a new window]   Fig. 3. (a) TEM-image of chrysotile (# 490), (b) TEM-image of (100) lattice fringes of antigorite (# 488), (c) HRTEM-image of antigorite (# 488) showing characteristic wavy layers, (d) TEM-image of (001) lattice fringes of antigorite (# 488).

View larger version (21K): [in this window] [in a new window]   Fig. 4a-h. Periodicity distribution of synthetic antigorites (see Table 1). P[GPa]/T[°C]/t[d]. Filled symbols: most frequent a periodicities.

View larger version (12K): [in this window] [in a new window]   Fig. 5a, b. Antigorite periodicity distributions for two samples obtained at the same P, T-conditions but for different run-durations. Symbols are the same as in Fig. 4a-h.

View larger version (18K): [in this window] [in a new window]   Fig. 6. P, T-dependence of the most frequent m-values as determined from the periodicity distribution (Fig. 4a-h and 5) of synthetic antigorite from different parageneses (forsterite + antigorite: squared numbers; brucite + antigorite: circled numbers). Based on these data, dotted lines limit P, T-ranges for different m-values (labelled by italic numbers), which decline according to reaction (5, see text). * indicates the m-value of a natural antigorite as determined by Mellini (e.g. (1987). Maximum stability fields of antigorite are shown as determined experimentally by Ulmer & Trommsdorff (1995) (U & T, 95) and Wunder & Schreyer (1997) (W & S, 97). The chrysotile to antigorite transformation is taken from Evans et al. (1976). The dashed line represents a theoretical P, T, t-path for the subduction of a 50 Ma old oceanic crust as modelled by Peacock (1990). Abbreviations: Ant, antigorite; Br, brucite; Fo, forsterite; A, phase A; W, water.