Groutite is a manganese oxyhydroxide, alpha-MnO(OH), and is isostructural with diaspore. The manganese is trivalent and coordinated with O to form edge-sharing Mn³⁺O₆ octahedra, which are linked three-dimensionally by sharing vertices. The three dimensional structure is comprised of tunnels, with the sizes of these tunnels determined by the chain widths. In groutite, the edge-sharing octahedra form double chains, whereas in manganite (gamma-MnO(OH); isostructural with rutile) the edge-sharing octahedra form single chains. Jahn-Teller distortions (Kohler et al., 1997) affect the octahedral shape with four short and two long Mn-O bond lengths and determine partially where the hydrogen links the octahedral chains to form the overall topologies. Groutite may be described as a distorted derivative of ramesdellite (MnO₂, with Mn⁴⁺ and a double octahedral chain; isostructural with gibbsite) and manganite as a distorted derivative of pyrolusite, beta-MnO₂ (and a single octahedral chain with Mn⁴⁺; isostructural with rutile). Feitknechtite, beta-MnO(OH), has not been well described. Pyrolusite occurs in low temperature hydrothermal deposits and as replacement after other Mn oxide minerals. Groutite and ramesdellite are rare, often altering to pyrolusite, and occur in low temperature hydrothermal deposits. Feitknechtite occurs as fine-grained mixtures with hausmannite.