Abstract: Macaulayite was found by the late I. Stephen (Soil Science Department, University of Aberdeen) in an outcrop of reddened, deeply weathered granite, near Inverurie, Aberdeenshire. It is blood red in colour, very fine-grained, and has refractive indices greater than 1.734. Its calculated density is 4.41 g/cm3. The mean of fourteen electron microprobe analyses in the anhydrous form is Fe2O3 84.67, Al2O3 4.01, SiO2 11.32%. With thermogravimetric data this leads to a formula of (Fe44.753+Al3.38)Si7.95O86(OH)4; the ideal formula is Fe243+Si4O43(OH)2. The cell indexes as C-centred monoclinic with a 5.038, b 8.726, c 36.342Å, β 92°. The strongest X-ray lines are 36.6 (vs), 18.16 (vs), 3.700 (25), 2.720 (35), 2.533 (100), 2.214 (20), and 1.420 (35). Macaulayite has a layer structure, thought to consist of a double hematite unit terminated on both sides by silicate sheets and with water between these sheets. The infra-red spectrum includes absorption bands at 3597, 1052, 1033, and 858 cm−1, arising from the hydroxysilicate component of the mineral and at 647, 520, 438, 400, 304, and 227 cm−1 corresponding to the platy hematite unit. The mineral is named for the Macaulay Institute for Soil Research and the name was approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association prior to publication.