The thermal behaviour of fluorapophyllite from the Mieedzyrzecze sill near Bielsko-Biala in the Polish Carpathians

Abstract

Fluorapophyllite, (K 0 . 9 6 Na 0 . 0 3 ) 0 . 9 9 Ca 3 . 9 6 (Si 7 . 9 5 Al 0 . 0 4 P 0 . 0 3 ) 8 . 0 2 O 1 9 . 9 8 (F 0 . 8 3 OH 0 . 1 7 ) 1 . 0 0 . 8.02 H 2 O has been found in the Miedzyrzecze sill. The apophyllite crystals were collected in the complex veins cross cutting small pectolite skarn body at the top of picrite sill. Apophyllite from Miedzyrzecze exhibits no signs of abnormal optical properties; it is tetragonal with lattice parameters: a = 8.974 (2) A, c = 15.798 (6) A. The dehydration behaviour of apophyllite from Miedzyrzecze was studied by means of X-ray diffraction, thermal analysis and infrared spectroscopy; also its dielectric properties were measured. The DTA curve shows two distinct endothermic peaks connected with irreversibly removal of water molecules from K + (OH 2 ) 8 polyhedron, centred at about 348 and 454 °C, respectively. All analytical data obtained confirm suggestion, that amorphous layers on the outer parts of apophyllite grains form a diffusion barrier causing moderation of H 2 O molecule migration and the second endothermic peak origin. The ring diffusion of H 2 O starts at about 160 °C, when the water molecules begin to migrate from the outside parts of apophyllite grains through 4- and 8- membered rings along [001] direction. Expulsion of the water molecule from the K, Ca sheets causes initiation of the non-diffracting amorphous phase origin. The subsequent water loss gives occasion to permanent distortion of sillicate layers with simultaneous K and Ca coordination collapse. The IR data obtained was useful to discuss the structural property of amorphous form of dehydrated apophyllite.