Effects of temperature on the crystal structure of epidote: a neutron single-crystal diffraction study at 293 and 1,070 K

The effects of temperature on the crystal structure of a natural epidote [Ca(1.925) Fe(0.745)Al(2.265)Ti(0.004)Si(3.037)O(12)(OH), a = 8.890(6), b = 5.630(4), c = 10.150(6) and beta = 115.36(5)A degrees, Sp. Gr. P2(1) /m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe(3+) at the M(3) site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)-H(1)center dot center dot center dot O(2) and O(10)-H(1)center dot center dot center dot O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)-H(1)center dot center dot center dot O(4) is energetically more favourable, as H(1)center dot center dot center dot O(4) = 1.9731(28) , O(10)center dot center dot center dot O(4) = 2.9318(22) and O(10)-H(1)center dot center dot center dot O4 = 166.7(2)A degrees, whereas H(1)center dot center dot center dot O(2) = 2.5921(23) , O(10)center dot center dot center dot O(2) = 2.8221(17) and O(10)-H(1)center dot center dot center dot O2 = 93.3(1)A degrees. The O(10)-H(1) bond distance corrected for "riding motion" is 0.9943 . The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe(3+) share the M(3) site along with Al(3+) [%Fe(M3)(1,070K) = 74(2)%]. The refined amount of Fe(3+) at the M(1) is not significant [%Fe(M1)(1,070K) = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for "rigid body motions". The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)-O(9)-Si(1) of the Si(2)O(7) group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)center dot center dot center dot O(4) and H(1)center dot center dot center dot O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe(3+) between different octahedral sites and/or convergent ordering on M(3) are discussed.