Original paper

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

DOI: 10.1127/0935-1221/2013/0025-2262

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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.


ferrisepiolitenew mineralclay mineralcoupled substitutioncopper skarnsaishitangchina