AbstractWilhelmgümbelite, ideally [ZnFe²⁺Fe₃³⁺(PO₄)₃(OH)₄(H₂O)₅]·2H₂O, is a new secondary phosphate mineral related closely to schoonerite, [ZnMnFe₂²⁺Fe³⁺(PO₄)₃(OH)₂(H₂O)₇]·2H₂O, from oxidized zones of the Hagendorf-Süd pegmatite, Hagendorf, Oberpfalz, Bavaria, Germany. Wilhelmgümbelite occurs as radiating sprays of needle-like rectangular laths, up to 0.2 mm long and with colour varying from light yellow brown to orange red. Cleavage is perfect parallel to {010}. The mineral is associated closely with an oxidized pseudomorph of phosphophyllite, recently named steinmetzite. Other associated minerals are albite, apatite, chalcophanite, jahnsite, mitridatite, muscovite and quartz. The calculated density of wilhelmgümbelite is 2.82 g cm^–3. It is optically biaxial (+) with α = 1.560(2), β = 1.669(2), γ = 1.718(2), 2V(meas) = 63(1)° and 2V(calc.) = 65°. Dispersion is weak with r > v, orientation X = b, Y = c, Z = a. Pleochroism is weak, with coloursZ = orange brown, Y = yellow brown, X = light yellow brown, Z >> Y > X. Electron microprobe analyses (average of seven analyses, seven crystals) with H₂O and FeO/Fe₂O₃ calculated on structural grounds, gave FeO 5.8, Fe₂O₃ 25.0, MnO 2.6, ZnO 16.4, P₂O₅ 28.7, H₂O 23.4, total 101.9 wt.%. The empirical formula, scaled to 3 P and OH^– adjusted for charge balance is Zn_1.50Mn_0.27²⁺Fe_0.60²⁺Fe_2.33³⁺(PO₄)³·(OH)_2.73(H₂O)_8.27. The structural formula is [Zn(Mn_0.27Fe_0.73³⁺)_∑1.0(Zn_0.25Fe_0.15²⁺Fe_0.60³⁺)_∑1.0(Zn_0.25Fe_0.45²⁺)_∑0.7Fe³⁺(PO₄)₃(OH,H₂O)₉]·2H₂O.Wilhelmgümbelite has orthorhombic symmetry, Pmab, Z = 4, with the unit-cell parameters of a = 10.987(7) Å, b = 25.378(13) Å, c = 6.387(6) Å and V = 1781(2) ų. The strongest lines in the powder X-ray diffraction pattern are [d_obs in Å(I_obs) (hkl)] 12.65 (100) (020); 8.339 (5) (120); 6.421 (14) (001); 6.228 (8) (011); 4.223 (30) (120) and 2.111 (7) (0 12 0). Wilhelmgümbelite is an oxidized form of schoonerite, with the Mn²⁺ replaced principally by Fe³⁺. Its structure differs from that of schoonerite in having the Zn partitioned between two different sites, one five-coordinated as in schoonerite and the other tetrahedrally coordinated. Wilhelmgümbelite also differs structurally from schoonerite in having partial occupation of one of the Fe sites, which appears to be correlated with the Zn partitioning.