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Composition and I4/m -P4₂/n phase transition in scapolite solid solutions

Department of Geology and Mineralogy, Kyoto University, Kyoto, 606-8502, Japan

Correspondence: ^* E-mail: seto{at}kueps.kyoto-u.ac.jp

Scapolite is a metamorphic aluminosilicate mineral that can be described by the general formula (Na, Ca, K)₄(Al, Si)₆Si 6O₂₄(Cl, CO₃, SO₄). Two common end-member s are called marialite (Na₄ClSi₉Al₃O₂₄) and meionite (Ca₄CO₃Si₆Al₆O₂₄). Variations in scapolite composition can be described by two independent substitutions, NaSi(CaAl)_–1 and NaCl(CaCO₃)_–1. Twenty eight natural scapolites in the present study exhibit a range of compositions from X_EqAn [(Al–3)/3] = 8%and X_Me [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_1.49SiCl_0.47 [Ca_1.44Al(CO₃)_0.43]_–1, Na_1.69SiCl_0.58 [Ca_1.55Al(CO₃)_0.50]_–1, Na_1.91SiCl_0.79 [Ca_1.75Al(CO₃)_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 P4₂/n. Under darkfield observation (g =hkl, h + k + l = odd) using a transmission electron microscope (TEM), the P4₂/n 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.

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