Water in diopside; an electron microscopy and infrared spectroscopy study

The hydroxyl contents of three wet diopside single crystals (H/Si = 700, 1200 and 3200 atomic ppm) and their behaviour after heating above 1000 degrees C have been studied by infrared spectroscopy and transmission electron microscopy. The infrared spectra of the as-received crystals show four absorption bands in the OH stretching region at 3640, 3535, 3460 and 3355 cm (super -1) , similar to the ones previously reported in the literature, plus another one at 3425 cm (super -1) which occurs only in a chromium-rich diopside crystal. Except for very thin clinoamphibole lamellae in one sample, these crystals contain no planar defects or inclusions. After annealing at 1100 degrees C under an argon flow, the absorption bands disappear within 25 to 70 hours. In the crystal containing amphibole lamellae, this phenomenon is accompanied by the precipitation of small water bubbles (diameter 100-3000 A) close to the amphibole lamellae which progressively disappear. The larger bubbles are connected to small sessile dislocation loops with Burgers vectors of 1/2[110] and l/2[ll0]. In the two other crystals, no precipitates accompany the loss of hydroxyls. It is suggested that H's of OH's were incorporated in diopside in the form of two types of point defects: the first one, related to the absorption band at 3640 cm (super -1) , has its O-H stretching direction parallel to the plane (101) (Beran, 1976) ; the second one, related to the four other absorption bands, has its stretching direction inclined at 30 + or - 5 degrees to the c axis and is probably related to trivalent (Al (super 3+) , Cr (super 3+) , Fe (super 3+) ) or monovalent (Na (super +) ) cationic substitutions. The dehydroxylation phenomenon occurring during annealing probably results from an oxidation process, similar to the one observed in iron-rich amphiboles, superimposed on a dehydrogenation process. In the crystal containing amphibole, the tiny water precipitates result from the breakdown of this amphibole into a clinopyroxene.