Guanacoite with high Mg content from the type locality (El Guanaco mine, Chile) was investigated to elucidate the crystal-chemical characteristics of this arsenate mineral. In this study, WDS X-ray intensity maps revealed that guanacoite single crystals from the investigated specimen show distinct and characteristic compositional zoning, which varies cyclically along , which is in the direction of the elongation of the prismatic crystals (Witzke et al. 2006); the boundaries separating different compositional regions lie perpendicular to the prism faces. The crystal structure of a selected fragment [a = 5.459(2), b = 16.808(9), c = 6.917(3) Å, β = 100.44(1)°, V = 624.1(5) Å3, P21/c, and Z = 2], was solved using direct methods and refined to an R index of 3.09% for 1385 observed [Io > 4σ(Io)] reflections measured at 103 K using MoKα X-radiation. The chemical formula based on the refinement is (Cu0.85Mg0.15)2Mg2(Mg0.83Cu0.17)(OH)4(H2O)4(AsO4)2. The structure model obtained is in good agreement with the previous structure determination, although the occupancies of the three M sites (M = Mg or Cu) are distinctly different, i.e., the sites are considerably richer in Mg, especially M1 and M3. These structural results suggest that guanacoite’s chemical formula should be enlarged from the Cu-rich composition Cu2Mg2(Mg,Cu)(OH)4(H2O)4(AsO4)2 to (Cu,Mg)2Mg2(Mg,Cu)(OH)4(H2O)4(AsO4)2 to include the Mg-rich members (Mg up to 3.395 apfu). It appears that the chemical variability of guanacoite is mainly due to Mg-Cu substitution on the M1 and M2 sites.