The structural properties of Sn precipitates in crystalline Si0.95Sn0.05 have been characterized by transmission electron microscopy and synchrotron x-ray diffraction. The Sn precipitates were produced by thermal annealing of epitaxial, single-crystalline Si1-xSnx layers grown by low-temperature molecular-beam epitaxy on Si (001) and relaxed Si1-xGex substrates. Two different phases of solid Sn were identified in the annealed layers: the semiconductor phase, alpha-Sn, and the metallic phase beta-Sn The precipitates were found to consist of either only beta-Sn or to contain crystallites of both solid Sn phases. The orientations, the sizes and the relative number densities of the alpha-Sn and beta-Sn crystallites were investigated. in situ heating and cooling experiments were performed in the transmission electron microscope to study the melting and solidification characteristics of the alpha-Sn and beta-Sn crystallites. The presence of alpha-Sn at temperatures far above the bulk alpha <--> beta transition temperature is explained by interface and pressure effects; the latter is likely to be due to the difference in thermal expansion of the precipitates and the matrix.