Ferrisepiolite: a new mineral from Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China

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 elastic 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: gamma' = 1.628(8), alpha' = 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 degrees C. The average chemical composition from wet chemistry, X-ray fluorescence spectrometry (XRF) and electron probe microanalysis (EPMA) is (Fe-1.84(3+),Fe-0.51(2+), Mg-1.56, Ca-0.05,Mn-0.02,Na-0.02)(Sigma=4) (Si-5.79,Fe-0.21(3+))(Sigma=6)O-15 center dot(O-1.60,OH0.40)(Sigma=2)center dot 6H(2)O for fibrous ferrisepiolite and (Fe-2.64(3+),Fe-0.80(2+),Mg-0.35,Ca-0.11, Mn-0.05,Na-0.05)(Sigma=4) (Si-5.18,Fe-0.82(3+))(Sigma=6)O-1.5 center dot(O-1.77,OH0.23)(Sigma=2)center dot 6H(2)O for earthy ferrisepiolite, which leads to the general formula: (Fe3+,Fe2+,Mg)(4) (Si,Fe3+)(6)O-15 center dot(O,OH)(2)center dot 6H(2)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 angstrom (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) angstrom, V = 1924.08 angstrom(3), Z = 4, Dcal = 2.51 g/cm(3) for fibrous ferrisepiolite and a = 13.638 (9) A, b = 27.011 (30) angstrom, c = 5.233(8) angstrom, V = 1927.58 angstrom(3), Z = 4, Dcal = 2.69 g/cm(3) for earthy ferrisepiolite. Ferrisepiolite is the Fe(III)-dominant analogue of sepiolite with the substitution of Fe3+ and/or Fe2+ for Mg in the octahedral sites, compensated by substitution of Fe3+ for Si4+ in the tetrahedral sites and O2- for OH- in the sites of structural hydroxyl, accompanied by a contraction of the structure along the c-axis and an expansion along the a-axis.