Ferrisepiolite: a new mineral from Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China
Xiangping, Gu; Xiande, Xie; Xiangbin, Wu; Guchang, Zhu; Jianqing, Lai; Kenich, Hoshino; Jiwu, Huang
European Journal of Mineralogy Volume 25 Number 2 (2013), p. 177 - 186
published: Mar 1, 2013
Open Access (paper can be downloaded for free)
Ferrisepiolite was discovered in the Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China, as latestage 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 (Fe3+1.84,Fe2+0.51,Mg1.56,Ca0.05,Mn0.02,Na0.02)Σ=4 (Si5.79,Fe3+0.21)Σ=6O15.(O1.60,OH0.40)Σ=2·6H2O for fibrous ferrisepiolite and (Fe3+2.64,Fe2+0.80,Mg0.35,Ca0.11,Mn0.05,Na0.05)Σ=4 (Si5.18,Fe3+0.82)Σ=6O15.(O1.77,OH0.23)Σ=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 [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 Å3, Z = 4, Dcal = 2.51 g/cm3 for fibrous ferrisepiolite and a = 13.638 (9) Å, b = 27.011 (30) Å, c = 5.233(8) Å, V = 1927.58 Å3, Z = 4, Dcal = 2.69 g/cm3 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.