The high-temperature behaviour of two natural chlorites has been studied by in situ X-ray diffraction: on a single-crystal of clinochlore, triclinic polytype IIb-4, space group C1̅, with pseudomonoclinic metric and composition (Mg9.14Fe2+1.02Fe3+0.01 Mn0.01Ti0.01Al1.76)∑=11.95 (Si6.32Al1.68)∑=8O20(OH)16; by powder diffraction for a chamosite with composition (Mg3.52Fe2+4.33 Fe3+1.15Al2.85)∑=11.85 (Si5.45Al2.55)∑=8O20(OH)16 and same symmetry. Unit-cell parameters were measured for both samples up to 550 °C. Diffraction data for structural refinement of clinochlore were collected at 25, 301, 399 and 502 °C. Room temperature 57Fe Mössbauer spectroscopy measurements were performed on the chamosite sample before and after heating, showing complete Fe oxidation above 500 °C. Mean thermal expansion coefficients of the two samples were: αa = 1.05(3) × 10−5, αb = 1.02(3) × 10−5, αc = 0.99(5) × 10−5 and αV = 3.07(7) × 10−5/°C for clinochlore and αa = 2.47(25) × 10−5, αb = 0.93(5) × 10−5, αc = 0.93(12) × 10−5 and αV = 4.38(25) × 10−5/°C for chamosite. These results confirm that the expansion of chlorite depends on the composition, but the dependence is not simply related to the Fe content, as suggested by previous studies.

The Si, Al tetrahedral volumes of clinochlore do not significantly change in the T range 25–550 °C. The expansion of M1, M2, M3 and M4 octahedra is 1.5, 1.9, 3.0 and 3.4 % respectively. The interlayer OH–O distances increase of about 1 %, the mean thermal expansion coefficient being 4.5 × 10−6/°C, indicating that the hydrogen bonds maintain their moderate strengths also at high temperature. The tetrahedral rotation angle α decreases from 6.38 to 3.34°.

A comparison with the modifications induced by pressure on the same sample of clinochlore confirms that the response of the structure to T and P is only approximately opposite. By combining the effects on cell volume, it is possible to formulate the following approximate equation of state: V = V0 (1 – 1.14 × 10−2 ΔP + 3.07 × 10−5ΔT), where P is in GPa and T in °Celsius. The equation is valid for a triclinic polytype of chlorite having clinochlore composition.

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