Thermoelasticity and high-T behaviour of anthophyllite

The thermoelastic behaviour of anthophyllite has been determined for a natural crystal with crystal-chemical formula ANa0.01B(Mg1.30Mn0.57Ca0.09Na0.04) C(Mg4.95Fe0.02Al0.03) T(Si8.00)O22W(OH)2 using single-crystal X-ray diffraction to 973 K. The best model for fitting the thermal expansion data is that of Berman (J Petrol 29:445-522, 1988) in which the coefficient of volume thermal expansion varies linearly with T as αV,T = a1 + 2a2 (T - T0): α298 = a1 = 3. 40(6) × 10-5 K-1, a2 = 5.1(1.0) × 10-9 K-2. The corresponding axial thermal expansion coefficients for this linear model are: αa,298 = 1.21(2) × 10-5 K-1, a2,a = 5.2(4) × 10-9 K-2; αb,298 = 9.2(1) × 10-6 K-1, a2,b = 7(2) × 10-10 K-2. αc,298 = 1.26(3) × 10-5 K-1, a2,c = 1.3(6) × 10-9 K-2. The thermoelastic behaviour of anthophyllite differs from that of most monoclinic (C2/m) amphiboles: (a) the ε1 - ε2 plane of the unit-strain ellipsoid, which is normal to b in anthophyllite but usually at a high angle to c in monoclinic amphiboles; (b) the strain components are ε1 ≫ ε2 ε3 in anthophyllite, but ε1 ~ ε2 ≫ ε3 in monoclinic amphiboles. The strain behaviour of anthophyllite is similar to that of synthetic C2/mANa B(LiMg) CMg5TSi8 O22W(OH)2, suggesting that high contents of small cations at the B-site may be primarily responsible for the much higher thermal expansion ⊥(100). Refined values for site-scattering at M4 decrease from 31.64 epfu at 298 K to 30.81 epfu at 973 K, which couples with similar increases of those of M1 and M2 sites. These changes in site scattering are interpreted in terms of Mn ↔ Mg exchange involving M1,2 ↔ M4, which was first detected at 673 K.