The crystal structure of rosasite, (Cu, Zn)(2)(CO3)(OH)(2), and mcguinnessite, (Mg,Cu)(2)(CO3)(OH)(2) have been determined from powder data. The two minerals are isostructural, with space group P2(1)/a and cell constants a=12.8976(3), b=9.3705(1), c=3.1623(1) angstrom, beta=110.262(3)degrees, V =358.54(2) angstrom(3), for rosasite and a=12.9181(4), b=9.3923(2), c=3.1622(1) angstrom, beta=111.233(3)degrees, V=357.63(2) angstrom(3) for mcguinnessite. The crystal structure refinements were lead up to R-p =7.51%, wR(p)=10.39% for rosasite and R-p =5.12%, wR(p)=6.22% for mcguinnessite. In both the two structures, the Cu coordination octahedron is distorted towards an elongated tetragonal bipyramid, whereas the Zn (in rosasite) and Mg (in mcguinnessite) coordination octahedra display an almost regular shape, their distortion being due to a partial occupancy of Cu. The carbonate group was refined as a rigid body, with a regular triangular geometry. Metal coordination octahedra polymerize through edge sharing to form octahedral "columns" and "ribbons", running along  and responsible for the acicular habit of these minerals. The structural relationships between rosasite and malachite are discussed.
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