PHASE-TRANSITIONS IN THE SERIES BORACITE-TREMBATHITE-CONGOLITE - PHASE-RELATIONS

The Sussex, New Brunswick, marine-evaporite-hosted berate deposits con tain boracite-group minerals of the solid-solution series Mg3B7O13Cl - Fe3B7O13Cl. Samples of boracite, trembathite and congolite have been characterized by electron-probe microanalysis and by high-temperature optical examination and X-ray powder diffraction. At 25 degrees C, the orthorhombic boracite structure (Pca2(1)) is stable for compositions from Mg3B7O13Cl to (Mg1.9Fe1.1)B7O13Cl, and the rhombohedral congolite structure (R3c) occurs for compositions ranging from (Mg1.9Fe1.1)B7O1 3Cl to Fe3B7O13Cl. Ar high temperatures, all specimens examined have t hr cubic boracite structure (<F(4)over bar 3c>). Cooling results in a first-order phase transition to the orthorhombic structure, and specim ens with more than 36 mol.% Fe3B7O13Cl undergo a further first-order p hase transition to the rhombohedral structure. The boundaries between the phases with cubic, orthorhombic and rhombohedral structures are li near in composition-temperature space. There is a marked discontinuity in the unit-cell volume of specimens rich in Mg at the phase transiti on from the cubic to thr orthorhombic structure; the positive volume s train is greatest in the Mg-rich samples. The high-temperature X-ray d iffraction data show that the cubic-to-orthorhombic phase transition i s first order in samples rich in Mg. but increasing Fe content results in a trend toward tricritical or second-order character. Higher Fe co ntents are associated with a decrease in the absolute value of the vol ume strain due to the phase transition. Although the symmetry-breaking strain due to the phase transition is very small, it is revealed by t he variation in peak width as a function of temperature in the X-ray p owder diffractograms.