X-RAY-POWDER DIFFRACTION PATTERNS AND PHASE RELATIONSHIP OF TRIDYMITE MODIFICATIONS

Calculated and observed X-ray powder patterns of three ambient temperature modifications of tridymite: L1 - T(o) (= ordered low-tridymite MC), L2 - T(D) (= disordered low-tridymite PO5/10) and L3 - T(o) (= ordered low-tridymite MX-1) are reported. Deviations in the diffractograms due to texture effect, stacking disorder and intergrowth of different modifications which complicate identification and indexing are discussed. Changes of unit cell dimensions due to thermal expansion and phase transitions were measured from room temperature to about 700-degrees-C. Near 160-degrees-C all ordered and slightly disordered tridymite polymorphs transform to monoclinic high tridymite H3 - T (OS) with non-integral modulation, at about 210-degrees-C to orthorhombic high tridymite H2 - T (OC) and finally at about 400-degrees-C to hexagonal high-tridymite H1 - T (HP). Different from pseudo-orthorhombic L2 - T(D) (PO5/10) the ordered tridymite modifications L1 - T(o) (MC) and L3 - T(o) (MX-1) form an additional metrically orthorhombic modification H4 - T(o) (OP) between 110 and 160-degrees-C. L3 - T(o) (MX-1) shows a further metrically orthorhombic phase H5 - T(o) between 70 and 105-degrees-C. Strongly disordered tridymite L2 - T(Ds) has only one phase transition at 170-degrees-C. All phase transitions are reversible. The transformations near 160, 110 and 70-degrees-C show a temperature hysteresis.