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

Gu Xiangping, Xie Xiande, Wu Xiangbin, Zhu Guchang, Lai Jianqing, Hoshino Kenich and Huang Jiwu
Ferrisepiolite; a new mineral from Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China
European Journal of Mineralogy (February 2013) 25 (2): 177-186


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: 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 (sub 1.84) (super 3+) , Fe (sub 0.51) (super 2+) ,Mg (sub 1.56) ,Ca (sub 0.05) ,Mn (sub 0.02) ,Na (sub 0.02) ) (sub Sigma =4) (S i (sub 5.79) ,Fe (sub 0.21) (super 3+) ) (sub Sigma =6) O (sub 15) .(O (sub 1.60) ,OH (sub 0.40) ) (sub Sigma =2) D6H (sub 2) O for fibrous ferrisepiolite and (Fe (sub 2.64) (super 3+) ,Fe (sub 0.80) (super 2+) ,Mg (sub 0.35) ,Ca (sub 0.11) ,Mn (sub 0.05) ,Na (sub 0. 05) ) (sub Sigma =4) (Si (sub 5.18) ,Fe (sub 0.82) (super 3+) ) (sub Sigma =6) O (sub 15) .(O (sub 1.77) ,OH (sub 0.23) ) (sub Sigma =2) .6H (sub 2) O for earthy ferrisepiolite, which leads to the general formula: (Fe (super 3+) ,Fe (super 2+) ,Mg) (sub 4) (Si,Fe (super 3+) ) (sub 6) O (sub 15) .(O,OH) (sub 2) .6H (sub 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 Aa (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) Aa, V = 1924.08 Aa (super 3) , Z = 4, Dcal = 2.51 g/cm (super 3) for fibrous ferrisepiolite and a = 13.638 (9) Aa, b = 27.011 (30) Aa, c = 5.233(8) Aa, V = 1927.58 Aa (super 3) , Z = 4, Dcal = 2.69 g/cm (super 3) for earthy ferrisepiolite. Ferrisepiolite is the Fe(III)-dominant analogue of sepiolite with the substitution of Fe (super 3+) and/or Fe (super 2+) for Mg in the octahedral sites, compensated by substitution of Fe (super 3+) for Si (super 4+) in the tetrahedral sites and O (super 2-) for OH (super -) in the sites of structural hydroxyl, accompanied by a contraction of the structure along the c-axis and an expansion along the a-axis.

ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 25
Serial Issue: 2
Title: Ferrisepiolite; a new mineral from Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China
Affiliation: Chinese Academy of Sciences, Guangzhou Institute of Geochemistry, Laboratory of Mineralogy and Metallogeny, Guangzhou, China
Pages: 177-186
Published: 201302
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 21
Accession Number: 2013-046197
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, geol. sketch map
N35°17'15" - N35°17'15", E99°47'15" - E99°47'15"
Secondary Affiliation: Central South University, CHN, ChinaBeijing Technology Center of Exploration for Nonferrous Metals, CHN, ChinaHiroshima University, JPN, Japan
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201329
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