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 (Fe1.843+, Fe0.512+,Mg1.56,Ca0.05,Mn0.02,Na0.02)∑=4(Si5.79,Fe0.213+)∑=6O15.(O1.60,OH0.40)∑=2·6H2O for fibrous ferrisepiolite and (Fe2.643+,Fe0.802+,Mg0.35,Ca0.11,Mn0.05,Na0.05)∑=4(Si5.18,Fe0.823+)∑=6O15.(O1.77,OH0.23)∑=2·6H2O for earthy ferrisepiolite, which leads to the general formula: (Fe3+,Fe2+,Mg)4(Si,Fe3+)6O15.(O,OH)2·6H2O. 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 [