FROM AMAZONITE RANDFEGMATITES OF WESTERN KEIVY TUNDRAS (KOLA PENINSULA)

摘要

Formanite-(Y) was found in amazonite randpegmatites of Ploskaya Mt. It occurs along fractures inside crystals of plumbomicrolite and forms also flattened prismatic crystals on their-surface. The mineral is transparent, weakly greenish or colorless. Natural formanite-(Y) is amorphous in X-rays. After 1 hour annealing at + 600 to 850 deg C the mineral shows tetragonal modification; at 900 deg C temperature it becomes monoclinic. Unit cell parameters of tetragonal and monoclinic modifications are, respectively: a = 5.12(1), c = 10.83(3) A and a = 5.033(3), b = 10.874 (6), c = 5.279(8) A, beta = 94.11 (7) deg, Typical formula of the mineral - ABO_4, where A = Y, REE, Ca, Th U; B = Ta, Nb, Ti, Fe. Chemical analyses show insignificant variations of their contents: (Y_(0.60)-0.81)Yb_(0.16)-0.30 REE_(0.29-0.49))_(1.05- 1.19)(Ta_(0.61-0.67)Nb_(0.23-0.30)Ti_(0.01-0.02))_(0.91-0.98)O_4. In spectrum of REE the light elements (LREE; are entirely absent and the HREE series has distinct ytterbium maximum. The phase heterogeneity of formanite-(Y) is caused by microinclusions of other mineral phases, mainly oxides and sulfides. In evolution of amazonite randpegmatites minerals crystallization of formanite-(Y) is connected with the later stages of Ta-Nb-mineralization reflecting the maximum activity of Ta and the beginning of Y-REE mineralization, namely its early (ytterbium) branch.