Composition and I4/m-P4_2 phase transition in scapolite solid solutions

摘要

Scapolite is a mctamorphic aluminosilicate mineral that can be described by the general formula (Na,Ca,K)_4(Al,Si)_6,Si6O_(24)(Cl,CO_3,SO_4). Two common end-members are called marialite (Na_4uClSi_9Al_3O_(24)) and meionite (Ca_4CO_3,Si_6,AL_6,O_(24)). Variations in scapolite composition can be described by two independent substitutions, NaSi(CaAl)_(-1) and NaCl(CaCO_3)_(-1). Twenty eight natural scapolites in the present study exhibit a range of compositions from X_(EqAn)[(Al-3)/3] = 8% and XMt [Ca/(Na+K+Ca)] = 7% to X_(EqAn) = 82% and X_(Me) = 90%. Several coupled exchange reactions can be identified in some inhomogeneous samples (e.g., Na_(1.49)SiCl_(0.47)[cA_(1.44)Al(CO_3)_(0.43)]_(-1), Na_(1.69)SiCl_(0.58)[Ca_(1.44)Al_(CO_3)_(0.43)_(-1)], Na_(1.69)SiCl_(0.58)[Ca_(1.55)Al(CO_3)_(0.50)_(-1), Na_(1.91)sIcL_(0.79)[Ca_(1.75)Al(CO_3)_(0.69)]_(-1). The extent of coupling between the two substitutions is controlled by the crystallization environment (P, T, and mineral assemblages). Electron diffraction patterns suggest that the symmetry of scapolite with X_(Me). up to 18% is I4/m, whereas that for intermediate scapolite from X_(Me) = 18% to at least X_(Me) = 90% is P4_2. Under dark-field observation (g = hkl, h + k + l = odd) using a transmission electron microscope (TEM), the P42 samples have anti phase domains of various sizes, the presence of which provides evidence for an I-P phase transition. A wide compositional range of scapolite solid solutions should have an I4/m symmetry at the time of formation.