Erythrite is the cobalt arsenate Co3(AsO4)2 8H2O which was named after its red color (Greek erythros) which may vary from purple to pink depending on the actual cobalt content and neighborhood (vide infra). Erythrite is the Co rich end member of a series with cobalt and nickel as the prevalent elements besides minor amounts of Fe and Mg. The nickel dominant end member is Annabergite. All share the same crystallography of the vivianite group (vide infra). Erythrite is often found as radiating tufts of tiny acicular, fibrous crystals. However, botryoidal or reniform aggregates or isolated spheres, sometimes spiky, are also common. The photographed specimen originates from the Bergkappe mine in Schneeberg, Saxony in Germany.
The artistic picture shows the unit cell of the crystal structure along the c-axis using the data from Wildner et al. (Eur. J. Mineral. 8 (1996) 187) who analyzed a sample from Schneeberg with the metal ratio Co:Fe:Ni = 1:0.37:0.12. Two different octahedral metal sites, isolated and edge sharing octahedra (blue), are linked through tetrahedral AsO4 (green, O = red). The layered material (a-c plane) is stacked along the b-axis and dashed lines are representing intra- and interlayer hydrogen bonding (H = white). Using Mössbauer spectroscopy Wildner et al. have demonstrated a preference of iron for the di-octahedral sites.