Ferrisepiolite was discovered in the Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China, as late-stage veinlets in copper-sulphide ores hosted in layered hedenbergite-andradite-actinolite skarn related to Indo-Sinian quartz diorite and Lower Permian metamorphosed clastic and carbonate rocks. Ferrisepiolite was formed in a highly oxidizing environment from low-temperature Fe-rich fluids and crystallized in cavities and fractures within the skarn-ore deposit. The mineral occurs in brown earthy and fibrous aggregates and shows brown to red-brown colour with strong pleochroism and 2nd order interference colours in a petrographic microscope. The measured refraction indices in white light for fibrous ferrisepiolite are: γ′ = 1.628(8), α′ = 1.592–1.620. The thermal analysis of ferrisepiolite reveals a lower dehydration temperature of structural hydroxyl than sepiolite and a small weight loss (0.1–0.9%) in the range 500–700 °C. The average chemical composition from wet chemistry, X-ray fluorescence spectrometry (XRF) and electron probe microanalysis (EPMA) is (Fe<sub>1.84sub><sup>3+sup>, Fe<sub>0.51sub><sup>2+sup>,Mg<sub>1.56sub>,Ca<sub>0.05sub>,Mn<sub>0.02sub>,Na<sub>0.02sub>)<sub>∑=4sub>(Si<sub>5.79sub>,Fe<sub>0.21sub><sup>3+sup>)<sub>∑=6sub>O<sub>15sub>.(O<sub>1.60sub>,OH<sub>0.40sub>)<sub>∑=2sub>·6H<sub>2sub>O for fibrous ferrisepiolite and (Fe<sub>2.64sub><sup>3+sup>,Fe<sub>0.80sub><sup>2+sup>,Mg<sub>0.35sub>,Ca<sub>0.11sub>,Mn<sub>0.05sub>,Na<sub>0.05sub>)<sub>∑=4sub>(Si<sub>5.18sub>,Fe<sub>0.82sub><sup>3+sup>)<sub>∑=6sub>O<sub>15sub>.(O<sub>1.77sub>,OH<sub>0.23sub>)<sub>∑=2sub>·6H<sub>2sub>O for earthy ferrisepiolite, which leads to the general formula: (Fe<sup>3+sup>,Fe<sup>2+sup>,Mg)<sub>4sub>(Si,Fe<sup>3+sup>)<sub>6sub>O<sub>15sub>.(O,OH)<sub>2sub>·6H<sub>2sub>O. The powder X-ray diffraction (XRD) of ferrisepiolite is in agreement with that of sepiolite. The eight strongest lines of the pattern of fibrous ferrisepiolite are [d in Å (I)(hkl)]: 12.163(100)(110), 4.298(35)(131), 3.751(15)(260), 3.394(29)(400), 3.198(13)(331), 2.561(45)(191), 2.436(31)(212), 2.260(14)(391). Powder XRD and single-crystal electron-diffraction data show that ferrisepiolite is isostructural to sepiolite with orthorhombic unit-cell parameters (space group Pncn) a = 13.619 (8), b = 26.959 (26), c = 5.241 (7) Å, V = 1924.08 Å<sup>3sup>, Z = 4, Dcal = 2.51 g/cm<sup>3sup> for fibrous ferrisepiolite and a = 13.638 (9) Å, b = 27.011 (30) Å, c = 5.233(8) Å, V = 1927.58 Å<sup>3sup>, Z = 4, Dcal = 2.69 g/cm<sup>3sup> for earthy ferrisepiolite. Ferrisepiolite is the Fe(III)-dominant analogue of sepiolite with the substitution of Fe<sup>3+sup> and/or Fe<sup>2+sup> for Mg in the octahedral sites, compensated by substitution of Fe<sup>3+sup> for Si<sup>4+sup> in the tetrahedral sites and O<sup>2−sup> for OH<sup>−sup> in the sites of structural hydroxyl, accompanied by a contraction of the structure along the c-axis and an expansion along the a-axis.