PHASE-TRANSITIONS IN THE SERIES BORACITE - TREMBATHITE - CONGOLITE - AN INFRARED SPECTROSCOPIC STUDY

The marine-evaporite-hosted berate deposits at Sussex, New Brunswick, contain boracite-group minerals of the solid-solution series Mg3B7O13C l - Fe3B7O13Cl. At 25 degrees C the orthorhombic structure (Pca2(1)) i s stable for compositions from Mg3B7O13Cl to (Mg1.9Fe1.1)B7O13Cl, and the rhombohedral structure (R3c) occurs for compositions ranging from (Mg1.9Fe1.1)B7O13Cl to Fe3B7O13Cl. At temperatures greater than 330 de grees C, crystals of all compositions have the cubic boracite structur e (F43c). Cooling results in a first-order phase transition to the ort horhombic structure, and specimens with more than 36 mol.% Fe3B7O13Cl undergo a further first-order phase transition to the rhombohedral str ucture. Samples of boracite, trembathite and congolite have been exami ned using both mid- and far-infrared absorption spectroscopy at 25 deg rees C and at high temperatures. This study suggests the existence of a previously unknown boracite-type phase in samples with Mg approximat e to Fe over the temperature interval similar to 90 to 210 degrees C. Phonons in the mid-infrared range are insensitive to composition, assu ming that no phase transition has occurred, indicating that they are m ainly due to vibrations in the berate framework. The far-infrared spec tra vary considerably with composition, thus the modes are largely due to vibrations of the metal atoms. Phase transitions are readily disce rnible in either middle- or far-infrared spectra. The peak that occurs at similar to 1350 cm(-1) is associated with an antisymmetric stretch ing mode of the BO3 triangles; this peak is observed in all samples at all temperatures, demonstrating that the BO3 triangles persist even i n the cubic structure. As X-ray-diffraction studies indicate that the cubic structure contains BO4 tetrahedra but not BO3 triangles, the pre sence of the modes due to BO3 triangles in the spectra corresponding t o material with the cubic structure may be due to a dynamic disorder o f boron and oxygen atoms, such that the average structure, as determin ed by X-ray diffraction, contains only BO4 tetrahedra.