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

Sepiolite is a magnesium phyllosilicate widely used in the pharmaceutical, fertilizer, construction and pesticide industries owing to its large surface area and microporosity. The substitution of Ni for Mg in the octahedral sites leads to the end-member falcondoite (Ni,Mg)₄Si₆O₁₅(OH)₂·6H₂O (Springer, 1976). However, high substitution of Fe for Mg, commonly observed in many other silicate minerals, is not often observed in sepiolite, and the iron analogue of sepiolite is not known. Fe-bearing sepiolites with various amounts of iron have been reported (Caillère, 1936; Bøggild, 1951; Brauner & Preisinger, 1956; Strunz, 1957; Preisinger, 1959; Semenov, 1969, Binzer & Karup-Møller, 1974; Garía-Romero & Suárez, 2010), but none has been approved as an independent mineral species by IMA-CNMNC because of lack of adequate data on chemical composition and crystal structure. Chukanova et al. (2002) reported an iron-rich analogue of sepiolite with the composition Ca_0.02(Fe_2.23Mn_1.06Mg_0.52Zn_0.17Ti_0.08)_∑4.06(Si_5.94Al_0.06)_∑6.00O₁₅[(OH)_1.74O_0.26]_∑2.00·nH₂O. which was probably an Fe(II)-dominant analogue of sepiolite as judged from the chemical formula and was considered as a valid unnamed mineral (Smith & Nickel, 2007).

During a field survey on the Saishitang copper deposit in Xinghai County, Qinghai Province, China in August of 2006, an abundance of black pitchy or brown materials in the ores were assumed to be wood fossils. However, subsequent laboratory examination revealed that this material is an iron silicate mineral with the XRD pattern comparable to sepiolite. After further studies, a proposal for new mineral status with the name ferrisepiolite was submitted to and approved by the IMA-CNMNC (2010-061). The name, ferrisepiolite, indicates its structural relationship to sepiolite and the dominance of Fe (III). The name was originally used by Strunz (1957) to describe Fe-bearing sepiolite with Fe₂O₃ (14.57%) and FeO (1.06%), which was previously reported by Bøggild (1951) as a new mineral with the name “gunnbjarnite”. Binzer & Karup-Møller (1974) presented data for a mineral with the name “ferrisepiolite” with Fe₂O₃ (9.98%) and FeO (1.18%). “Xylotile” and “mountain wood” were also used to describe sepiolite with considerable Fe₂O₃ + FeO contents but less than 21.7% dominated by ferric iron (Caillère, 1936; Brauner & Preisinger, 1956). Occurrences of sepiolite with high contents of Fe and Mn in names “Mn-ferrisepiolite” and “iron-rich analogue of sepiolite”, probably with dominant ferrous iron in octahedral sites, were also reported by Semenov (1969) and Chukanova et al. (2002). Type material of ferrisepiolite is deposited in the collections of the Geological Museum of China with catalogue number M11786.

The Saishitang copper deposit is located on the Tibet-Qinghai Plateau at 99°47′15″E, 35°17′15″N with an altitude of 3600–4000 m, about 230 km southwest of Xining, the capital city of Qinghai Province. The deposit was formed as a result of the interaction of Indo-Sinian granitic intrusions with Permian metamorphosed clastic and carbonate rocks. The copper orebodies, with total copper metal reserve of about 374,000 tons (on-site mine data), are closely associated with layered skarn distributed along the Lower Permian strata (Fig. 1). The deposit is currently mined by West Mining Co. of China on three levels at 3350 m, 3400 m and 3450 m.

Ferrisepiolite is easily observed in copper orebodies on all three production levels and in drill cores. It occurs as veinlets, 2 mm to 1 cm in thickness, crosscutting and replacing chalcopyrite, pyrrhotite, pyrite, sphalerite and galena hosted by hedenbergite–andradite–actinolite–vesuvianite skarn. Rounded grains of calcite and siderite are also present as inclusions in ferrisepiolite. In contrast, ferrisepiolite is not observed in Permian strata and in neighbouring quartz diorite. According to the mine geologists, the recovery of copper in the processing mill is sometimes decreased by high abundance of ferrisepiolite in the ores. The paragenetic relationships of ferrisepiolite with other minerals show that the late-stage 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.

Ferrisepiolite occurs in two forms of aggregates: (a) as a brown earthy mass consisting of individual grains of flaky-acicular microlites with poor crystallinity, a few micro-metres or less in size(Fig. 2a, b) as a brown fibrous aggregate with an appearance of wood bark, consisting of fibrous crystals several micrometres in width and more than several centimeters in length with good crystallinity (Fig. 2b). Both forms of ferrisepiolite are not magnetic based on a hand magnet test and they do not dissolve in dilute hydrochloric acid. Crystals are low in hardness at 2–2.5 and they have a brown streak. The crystallinity of ferrisepiolite is apparently related to the chemical composition. Poorly-crystallized earthy ferrisepiolite has high Fe₂O₃ + FeO contents (42.64%) and low MgO contents (1.78%), whereas well-crystallized fibrous ferrisepiolite is lower in Fe₂O₃ + FeO (average 27.30%) and higher in MgO (8.61%) (Table 1).

In reflected light, brown earthy ferrisepiolite in polished section is featured by low reflectance, weak anisotropism, and networked microfissures (Fig. 3a, b), whereas fibrous ferrisepiolite shows clear bireflectance and strong anisotropism. In polarized transmitted light, brown earthy ferrisepiolite in thin section shows brown colour with weak pleochroism with 1st order yellow to red interference colour (Fig. 3c, d), whereas fibrous ferrisepiolite is distinctly pleochroic from light red brown (for light vibrating perpendicular to the fibre axis) to dark red brown (for light vibrating parallel to the fibre axis). Although masked by the strong absorption colour, its birefringence is appreciable and exhibits up to 2nd order interference colours in thin sections of ~30 μm thickness (Fig. 3e, f). Attempts to measure refractive indices from brown earthy ferrisepiolite were difficult because of poor crystallinity and insufficient size. Small chips of fibrous ferrisepiolite were extracted for measurements of refractive indices in white light by the oil immersion method with the results: γ′ = 1.628(8), α′ = 1.592–1.620, γ||c, α (or β)||a, which are appreciably higher than those of sepiolite (1.515–1.529) (see http://www.handbookofmineralogy.org/pdfs/sepiolite.pdf) and Fe-and Mn-bearing sepiolite (1.557–1.597) (Chukanova et al. 2002).

The compatibility index (CI) calculated from measured refractive indices and the chemical composition of fibrous ferrisepiolite is equal to 0.02, which is considered an “excellent” value showing the consistency of the variation of refractive indices with chemical compositions (Mandarino, 1976, 1979, 1981).

The differential thermal analysis (DTA) and thermogravimetric (TG) data of ferrisepiolite were obtained from a Netzsch STA 490PC thermoanalyzer in the temperature range of 30–1000 °C at 10 K/min in an open container with nitrogen atmosphere. Three endothermic stages are shown in the DTA curve of ferrisepiolite (Fig. 4). The range 30–200 °C (peak at 102–130 °C) represents the loss of hygroscopic and zeolitic water accounting for about 12.30–15.7% of the total weight, the shoulder in the range 200–500 °C (centre at 273–290 °C) represents the loss of bound H₂O for about 4.59–4.95% of the total weight and the range 500–700 °C (peak at 607 °C) represents loss of bound hydroxyl for 0.1–0.9% of the total weight. The exothermic valley at 745–755 °C represents the decomposition of ferrisepiolite into magnetite, hematite, quartz and/or pigeonite as confirmed by XRD analysis of the product. The sharp valley at 93–115 °C in the differential TG curve may indicate the change from losing hygroscopic water to zeolitic water. The subtraction of H₂O contents in chemical analyses (Table 1) from the total mass losses in TG analyses, 20.70 wt% and 18.20 wt%, yields the mass loss of hygroscopic water, 6.30 wt% and 2.96 wt% respectively for earthy and fibrous ferrisepiolite. The higher hygroscopic water contents in earthy ferrisepiolite (6.30 wt%) than fibrous ferrisepiolite (2.96 wt%) is attributed to the formation of internal networked microfissures as shown in Fig. 3a, b.

Two or more stages of dehydration of bound water in the range of 200–500 °C, reported in sepiolite (Preisinger, 1959; Nagata et al. 1974, Yang & Zhang, 1994; Frost et al. 2009), are not observed in ferrisepiolite. The dehydration temperature of structural hydroxyl below 700 °C for ferrisepiolite is apparently lower than that of sepiolite (around 810–830 °C, e.g., Frost et al. 2009), which may indicate that the bonding strength of hydroxyl is lower in ferrisepiolite. The mass loss of earthy ferrisepiolite in the range 500–700 °C is only ~0.1%, although a clear endothermic effect occurs in the DTA curve, suggesting the deficiency of hydroxyl in the crystal structure.

The chemical composition of ferrisepiolite was determined separately by wet analysis, wavelength-dispersive X-ray fluorescence spectrometer (XRF) and electron probe microanalyzer (EPMA) methods (Table 1). Wet analyses were performed at the Changsha Research Institute of Mines and Metallurgy according to China National Standard Methods for bulk silicate analysis (GB/T 14506.1~30-2010). XRF analysis was performed on pressed powder plate with a Rigaku Primus II spectrometer at 50 kV and 60 mA by means of the fundamental parameters (FP) method included in Rigaku software SQX. The EPMA analyses were performed with a JEOL JXA-8100 at 15 kV and 20 nA with the routine JEOL program. The back-scattered electron (BSE) image was used to determine the homogeneity of the composition (Fig. 3b). The partitioning of ferrous and ferric iron in XRF and EPMA analyses was calculated according to the weight ratio of Fe³⁺/Fe²⁺ (respectively equal to 4.32 and 4.05 for earthy and fibrous ferrisepiolite) as determined in wet analyses.

The calculated contents of H₂O in Table 1 are based on the assumption for the general formula of ferrisepiolite as:

or equivalently,

where y and z are determined from the metal oxide contents in the analyses by setting the total cation number equal to 10. The x value is determined as x = 2*(17-O) from the total oxygen atom number O = (17 − x/2) according to formula (2). The molecular number of H₂O is obtained from x value as 6 + x/2 and the weight percent of H₂O is obtained by recalculating formula (2) with the result of 13.96–15.53% (Table 1). The results are in good agreement with the values of H₂O obtained by wet analyses and also similar to the weight loss of ferrisepiolite in the range of 80–700 °C of the thermal gravimetric data of ferrisepiolite.

The average chemical compositions of earthy and fibrous ferrisepiolite are, respectively, (Fe_2.64³⁺,Fe_0.80²⁺,Mg_0.35,Ca_0.11,Mn_0.05,Na_0.05)_∑=4(Si_5.18,Fe_0.82³⁺)_∑=6O₁₅.(O_1.77,OH_0.23)_∑=2· 6H₂O and (Fe_1.84³⁺,Fe_0.51²⁺,Mg_1.56,Ca_0.05,Mn_0.02,Na_0.02)_∑=4(Si_5.79,Fe_0.21³⁺)_∑=6O₁₅.(O_1.60,OH_0.40)_∑=2·6H₂O. The substitution of Fe³⁺ and Fe²⁺ for Mg in sepiolite Mg₄Si₆ O₁₅(OH)₂·6H₂O is compensated by substitution of Si⁴⁺ by Fe³⁺ and, mostly, of OH⁻ by O²⁻. The general formula may be written as (Fe³⁺, Fe²⁺, Mg)₄(Si, Fe³⁺)₆O₁₅.(O, OH)₂· 6H₂O, which involves Fe³⁺ predominating over Fe²⁺ or Mg²⁺ in the octahedral sites and O²⁻ predominating over OH⁻ in the hydroxyl site with OH as low as 0.23. There is also appreciable Fe³⁺, as much as 0.21–0.82, substituting for Si⁴⁺ in the tetrahedral sites. The atom numbers of Fe³⁺ in octahedral sites, i.e. 2.60–2.68 for earthy ferrisepiolite and 1.71–1.96 for fibrous ferrisepiolite, and Fe³⁺ in tetrahedral sites, i.e. 0.70–0.96 for earthy ferrisepiolite and 0.09–0.36 for fibrous ferrisepiolite, are apparently greater than those of sepiolite in the literature, in which the atom numbers of Fe³⁺ in octahedral and tetrahedral sites were respectively less than 1.20 and 0.52 (Brauner & Preisinger, 1956; Strunz, 1957; Preisinger, 1959; Binzer & Karup-Møller, 1974; García-Romero & Suárez, 2010). Moreover, the composition of earthy ferrisepiolite may suggest the possibility of a dehydroxylated Fe(III) end-member of sepiolite with the formula of

Based on the chemical compositions of ferrisepiolite given in Table 1, the content (weight percent) of zeolitic water, bound H₂O, and structural hydroxyl (released in the form of H₂O) are 14.33%, 9.22% and 0.28% for earthy ferrisepiolite, 9.90%, 4.95% and 0.49% for fibrous ferriseiolite. These results are comparable to the mass loss of ferrisepiolite in the temperature ranges of 80–200 °C, 200–500 °C, and 500–700 °C as shown in Fig. 4.

Powder samples of earthy and fibrous ferrisepiolite were measured for X-ray diffraction on a Rigaku D/Max 2500 diffractometer at 40 kV and 250 mA (CuKα) with a scanning speed 0.02 °/s. Oriented polished sections of fibrous ferrisepiolite, respectively parallel and perpendicular to the fibre direction, were measured on Rigaku D/Max Rapid IIR microdiffractometer at 40 kV and 250 mA (CuKα) with the collimator of 0.05 mm and 1 h exposure. Silicon powder was also measured at the same conditions and the data were used for calibration. Samples of sepiolite of low iron contents (total iron < 1 wt%) from Ningxiang and Xiangtan of Hunan Province were also measured for comparison. Representative XRD patterns are shown in Fig. 5 and the peak data are given in Table 2. The patterns and peak data are in good agreement with those of sepiolite from ICDD pdf No. 13-0595 and 26-1226, suggesting ferrisepiolite is isostructural with sepiolite, probably with space group Pncn (Preisinger, 1959; Brindley, 1959; Nagata et al. 1974). The diffraction lines are indexed according to the calculated powder pattern from the crystal structure data of Brauner & Preisinger (1956) and Post et al. (2007). The (hk0) and (h00) reflections in polished sections parallel to the fibre are absent in polished sections perpendicular to the fibre, indicating that the elongation of the fibre is [001] (the c-axis). Sixteen strong lines were selected for refinement of unit-cell parameters using the method and program of Holland & Redfern (1997), with the results a = 13.638 (9), b = 27.011 (30), c = 5.233 (8) Å, V = 1927.58 ų, Z = 4 for earthy ferrisepiolite and a = 13.619 (8), b = 26.959 (26), c = 5.241 (7) Å, V = 1924.08 ų, Z = 4 for fibrous ferrisepiolite.

In comparison to the unit-cell data of sepiolite from Nagata et al. (1974); Post et al. (2007) and Sánchez del Río et al. (2011): a = 13.224–13.618 (average 13.372 Å), b = 26.731–27.383 (average 27.005 Å), c = 5.266–5.281 (average 5.276 Å), V = 1890.17–1911.89 (average 1905.06 ų), ferrisepiolite has a smaller c parameter but larger a parameter and cell volume V, indicating that the coupled substitution of Fe³⁺ and/or Fe²⁺ for Mg²⁺; Fe³⁺ for Si⁴⁺; and O²⁻ for OH⁻ may be accompanied by unit-cell contraction along the c-axis and expansion along the a-axis with an increase of the cell volume. It is also noted that the (040) reflection at 6.7 Å, commonly seen in sepiolite, is not observed in ferrisepiolite, which may be related to the substitution of Fe³⁺ and Fe²⁺ for Mg²⁺ and Si⁴⁺, as also shown by Binzer & Karup-Møller (1974).

The powder XRD data of ferrisepiolite are also comparable to those of falcondoite (Ni,Mg)₄Si₆O₁₅(OH)₂·6H₂O and loughlinite Na₂Mg₃Si₆O₁₆·8H₂O (Springer, 1976; Fahey et al. 1960). These minerals are classified into the palygorskite–sepiolite group of modulated-layer phyllosilicate with joined strips.

Powder samples of earthy and fibrous ferrisepiolite were placed in glass vials with 10 ml ethanol and sonicated in a bath for 10 min to produce a powder suspension. Drops of the suspensions were deposited on copper grids (300 mesh) coated with carbon film. The dried copper grids were observed on a Fei Tecnai G2-ST electron transmission microscope at 200 kV and well-shaped crystals were selected for electron diffraction as shown in Fig. 6.

The TEM images show that earthy ferrisepiolite consists principally of poorly crystallized, irregularly rounded grains mixed with acicular crystals of 20–30 nm width. Fibrous ferrisepiolite is mainly composed of fragmented fibrous crystals and irregular grains 10–50 nm in width. The electron diffraction patterns of euhedral crystals in both samples feature the 110 refection, in addition to other reflections of the lattice as displayed by Brindley (1959) and Sánchez del Río et al. (2011).

The authors thank Mr. Zhang Yunsheng and Mr. Ding Tianzhu of Saishitang Copper Mine for the help in field work. Thanks are also given to Prof. Huang Baogui of Changsha Research Institute of Mines and Metallurgy for wet chemical analysis, and to Prof. Chen Ming and Dr. Chen Linli of Guangzhou Institute of Geochemistry, CAS, respectively for constructive advice and for EPMA analysis. The manuscript was much improved by Dr. S. Guggenheim and an anonymous reviewer. The study is supported by two grants from National Science Foundation of China, grant No. 40872046, 41172042.