Unit-cell constants of a strontian-piemontite sample from St. Marcel (Val d'Aosta, Italy) with composition Ca1.80Sr0.20Al1.91Fe0.33Mn0.82Si3O12OH were measured at several temperatures by single-crystal X-ray diffractometry (MoKα radiation). Values at 25 and 800 °C are a = 8.884(3), b = 5.684(1), c = 10.202(3) Å, 0 = 115.23(2)°; and a = 8.934(9), b = 5.727(4), c = 1/130(1) Å, β = 115.26(9)°, respectively, for space group P21/m. Principal coefficients of thermal expansion are 0.72 × 10−5, 0.98 × 10 5, and 1.29 × 10−5 K−1. Intensity data were measured at 25 and 800 °C, respectively, and the corresponding least-squares refinements of the crystal structure converged to R = 0.041 and 0.056. The ther-mal-expansion ellipsoid is symmetrically oriented with respect to a and c. A significant shrinking of the Si2O7 group, with opening of the Si-O-Si angle, is observed at high temperature. A large and very anisotropic mean expansion (1.4 × 10−5 K−1) occurs for the (Mn,Fe) coordination octahedron [M(3) site]. The relation between thermal expansion of the lattice and thermal expansion of the coordination polyhedra is accounted for by a correlated tilting of polyhedra at high temperature. No deprotonation can be detected before the breakdown of the structure (880 °C); the dehydration products have been identified.