Synthetic ferric sulfate trihydrate, Fe₂(SO₄)₃·3H₂O, a new ferric sulfate salt

Ferric sulfate trihydrate has been synthesized at 403 K under hydro­thermal conditions. The structure consists of quadruple chains of [Fe₂(SO₄)₃(H₂O)₃] parallel to [010]. Each quadruple chain is composed of equal proportions of FeO₄(H₂O)₂ octa­hedra and FeO₅(H₂O) octa­hedra sharing corners with SO₄ tetra­hedra. The chains are joined to each other by hydrogen bonds. This compound is a new hydration state of Fe₂(SO₄)₃·nH₂O; minerals with n = 0, 5, 7.25-7.75, 9 and 11 are found in nature.

S1. Comment

Ferric sulfate trihydrate belongs to a group of ferric sulfates sharing a general formula Fe₂(SO₄)₃.nH₂O, that includes mikasaite (n = 0) (Miura et al., 1994), lausenite (n = 5) (Majzlan et al., 2005), kornelite (n = 7.25–7.75) (Ackermann et al., 2009; Robinson & Fang, 1973), paracoquimbite (n = 9) (Robinson & Fang, 1971), quenstedtite (n = 11) (Thomas et al., 1974) and a synthetic Fe₂(SO₄)₃ (n = 0), a polymorph of mikasaite (Christidis & Rentzeperis, 1975). These ferric sulfate minerals are usually found as efflorescence in acid mine drainage (AMD) areas, formed as oxidation products of pyrite (FeS₂) and other sulfide minerals (Jambor et al., 2000). Other commonly found ferric sulfates in AMD regions include coquimbite [(Fe,Al)₂(SO₄)₃.9H₂O], jarosite [KFe₃(SO₄)₂(OH)], rhomboclase [(H₅O₂)Fe(SO₄)₂.2H₂O] and copiapite [Fe²⁺Fe³⁺₄(SO₄)₂(OH)₂.20H₂O]. Dissolution of these minerals greatly increases water acidity because of ferric ion hydrolysis. Plus, these ferric sulfates usually absorb or co-precipitate with toxic metals such as Cr and Pb, and act as a secondary source of these toxic metal pollutants in local waters as they dissolve (Jambor et al., 2000). To better control and reduce the adverse environmental effects of ferric sulfates require a thorough understanding of the behavior and stability of these minerals as functions of environmental factors, such as pH, relative humidity and temperature. Understanding the transformations of the ferric sulfates as a function of environmental conditions has been the focus of recent studies (Ackermann et al., 2009; Majzlan, 2010; Tosca et al., 2007; Xu et al., 2009, 2010).

One difficulty in delineating phase stability relationships in the Fe^III–SO₄–H₂O system is the need for correct and complete crystal structure models for all phases in the system. For example, paracoquimbite and coquimbite had for a long time been considered as polymorphs (Fang & Robinson, 1970) until a recent study showing the amount of aluminium in the mineral determines the structure type: paracoquimbite is pure Fe₂(SO₄)₃.9H₂O, while coquimbite has an Al/Fe ratio close to 1/3 with Al predominantly occupying the [M(H₂O)₆]³⁺ metal site (Majzlan et al., 2010). A third structure type in this coquimbite series was recently found in a mineral with a 1:1 Al/Fe ratio (Demartin et al., 2010). Further, synthesis of ferric sulfates often produces new phases (Chipera et al., 2007; Freeman et al., 2009; Majzlan et al., 2005; Peterson et al., 2009; Xu et al., 2009). Structures of these new phases are mostly unresolved because of their occurrence only as fine-grained mixed-phase powders. During the course of our survey of the Fe^III–SO₄–H₂O system we discovered a new phase, ferric sulfate trihydrate.

The structure of trihydrate contains identical quadruple chains of [Fe₂(SO₄)₃(H₂O)₃]_∞⁰ parallel to [010], as shown in Fig. 1. Each quadruple chain consists of four single chains of alternating FeO₆ octahedra and SO₄ tetrahedra extending along the b axis. Of the four single chains, two are symmetrically independent as `–Fe(1)—S(1)—Fe(1)–' and `–Fe(2)—S(2)—Fe(2)–'; the other two are generated through an inversion center. A third sulfate group S(3)O₄ connects to Fe(1)O₆ and Fe(2)O₆ by corner sharing. Of the three unique water molecules, one (O1W) is coordinated to the Fe(1) site, and the other two (O2W and O3W) are coordinated to the Fe(2) site.

The quadruple chains are linked to one another by water–sulfate O—H···O hydrogen bonds (Table 2). All hydrogen bonds involve terminal O in SO₄ groups as the acceptor, except the O1W—H1B···O3ⁱⁱ [symmetry code: (ii) -x, y - 1/2, -z + 1/2] hydrogen bond, where the acceptor O bridges Fe(1) and S(1). There is also one intramolecular hydrogen bond, O1W—H1B···O8.

The quadruple chain structure of the ferric sulfate trihydrate has no similar counterparts in any known sulfates. Though unique, the trihydrate structure shares similarities to other phases in the Fe₂(SO₄)₃.nH₂O group. For example, the FeO₆ octahedra do not directly connect with each other but are linked via corner-sharing SO₄ tetrahedra. It should be mentioned that FeO₆ octahedra do share corners in some other ferric sulfates such as copiapite and jarosite.

Including the trihydrate described here, there are a total of six hydration states of Fe₂(SO₄)₃.nH₂O; namely n = 0, 3, 5, 7.25–7.75, 9 and 11. Anhydrous ferric sulfate has two polymorphs, a monoclinic form and a trigonal form. The trigonal form occurs in nature as the mineral mikasaite, while the synthetic monoclinic form has no mineral equivalent (Christidis & Rentzeperis, 1975; Miura et al., 1994). Both forms consist of frameworks of connected FeO₆ octahedra and SO₄ tetrahedra. The structure of lausenite, Fe₂(SO₄)₃.5H₂O, is composed of corrugated slabs of [Fe₂(SO₄)₃(H₂O)₅]_∞⁰ (Majzlan et al., 2005). The five water molecules shown in the formula are all coordinated to the two Fe sites, two coordinated to one and three coordinated to the other. The structure of kornelite, Fe₂(SO₄)₃.7.25–7.75H₂O, consists of slabs similar to the ones in lausenite, with a formula [Fe₂(SO₄)₃(H₂O)₆]_∞⁰. Each of the two Fe sites is bonded to three water molecules. The remaining 1.25 to 1.75 water molecules shown in the formula are located between neighboring slabs as isolated water (Robinson & Fang, 1973). The paracoquimbite structure contains isolated clusters of [Fe(H₂O)₆]³⁺ and [Fe₃(SO₄)₆(H₂O)₆]^3- and six uncoordinated water molecules (Robinson & Fang, 1971). The structure of quenstedtite, Fe₂(SO₄)₃.11H₂O, consists of isolated clusters of [Fe(SO₄)(H₂O)₅]⁺ and [Fe(SO₄)₂(H₂O)₄]^- and two uncoordinated water molecules (Thomas et al., 1974).

As the hydration state increases, FeO₆ octahedra and SO₄ tetrahedra tend to be disassociated, as more water molecules coordinate to Fe to form simple clusters. The hydration state of trihydrate lies in between that of anhydrous ferric sulfate and lausenite. Like lausenite, the structure of the trihydrate does not have uncoordinated water molecules as found in kornelite and higher hydration states. As opposed to limited clusters in the higher hyration states, both the trihydrate and the lausenite have infinite clusters, one-dimensional chains for the trihydrate and two-dimensional slabs for the lausenite. Further, the quadruple chain of trihydrate has three terminal O sites for water molecules, while the slab of lausenite has five. This appears to be counterintuitive: a common first impression would be that a chain arrangement has more terminal O sites than two-dimensional sheets. It is easy to understand this if one considers the fact that a quadruple chain has much less [far fewer] terminal sites than four single chains; also, the slab in lausenite is far from [less like] a dense sheet such as the MnO₂ layer in birnessite, [and closer to] but closer to a net of interlaced chains.

Therefore, the trihydrate, though having a unique quadruple chain structure, shares the basic structural features present in other phases in the Fe₂(SO₄)₃.nH₂O system. The trihydrate structure also fits in the trend of structural changes set by hydration levels.

Related literature

For related literature, see: Ackermann et al. (2009); Chipera et al. (2007); Christidis & Rentzeperis (1975); Demartin et al. (2010); Fang & Robinson (1970); Freeman et al. (2009); Jambor et al. (2000); Majzlan (2010); Majzlan et al. (2005); Majzlan, Dordevic, Kolitsch & Schefer (2010); Miura et al. (1994); Peterson et al. (2009); Robinson & Fang (1971, 1973); Thomas et al. (1974); Tosca et al. (2007); Xu et al. (2009, 2010).

S2. Experimental

α-Fe₂O₃ [1.000 (1) g] and sulfuric acid [1.939 (1) g] with a nominal concentration of 95.9 wt% H₂SO₄ were mixed in a 23 ml Teflon-lined vessel, sealed in a Parr stainless steel autoclave then stored in an isotemp oven set at 403 K for 9 d. The product was an inhomogeneous, yellowish pink, solid block. Examination with optical microscopy revealed transparent yellow crystals embedded in a pink powdery matrix. The yellow crystals are cuboids or short rectangular prisms, 100 to 300 µm on the longest edge and 40 to 100 µm on the shortest edge. The structure of the yellow crystal was determined from single-crystal X-ray diffraction to be the ferric sulfate trihydrate presented above. The pink matrix was a combination of lausenite and rhomboclase identified from powder X-ray diffraction (supporting material).

S3. Refinement

H atoms were found in difference Fourier maps and subsequently placed in idealized positions with constrained distances of 0.90 Å (O—H), and with U_iso(H) = 1.2U_iso of attached oxygen atoms.

Computing details

Data collection: SMART (Bruker, 1998); cell refinement: SAINT-Plus (Bruker, 1998); data reduction: SAINT-Plus (Bruker, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: software?; software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

Fig. 1. The ferric sulfate trihydrate structure, as viewed down the b axis. FeO6 octahedra are brown, SO4 tetrahedra are yellow, H atoms are white. Dotted lines denote hydrogen bonds.

diiron(III) trisulfate trihydrate

Crystal data

Fe2(SO4)3·3H2O	F(000) = 904
Mr = 453.93	Dx = 2.656 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 855 reflections
a = 11.281 (3) Å	θ = 1.9–27.1°
b = 6.336 (2) Å	µ = 3.20 mm^−1
c = 16.278 (5) Å	T = 298 K
β = 102.676 (8)°	Block, yellow
V = 1135.1 (6) Å^3	0.12 × 0.12 × 0.06 mm
Z = 4

Data collection

Bruker SMART CCD diffractometer	2309 independent reflections
Radiation source: fine-focus sealed tube	1894 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.033
ω and ϕ scans	θmax = 26.4°, θmin = 1.9°
Absorption correction: multi-scan (SADABS; Bruker, 2001)	h = −13→14
Tmin = 0.700, Tmax = 0.831	k = −7→7
7266 measured reflections	l = −20→20

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.028	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.084	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.045P)^2 + 0.7177P] where P = (Fo^2 + 2Fc^2)/3
2309 reflections	(Δ/σ)max < 0.001
199 parameters	Δρmax = 0.49 e Å^−3
6 restraints	Δρmin = −0.55 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Fe1	0.02030 (3)	0.25527 (5)	0.11807 (2)	0.01248 (13)
Fe2	0.36121 (3)	0.77262 (6)	0.09097 (2)	0.01340 (13)
S1	0.07269 (6)	0.75034 (9)	0.09067 (4)	0.01171 (16)
S2	0.31145 (6)	0.27291 (10)	0.11913 (4)	0.01403 (16)
S3	0.26506 (6)	0.72726 (10)	−0.11735 (4)	0.01643 (17)
O1	0.20241 (17)	0.7803 (3)	0.12936 (11)	0.0172 (4)
O2	0.05876 (18)	0.6955 (3)	0.00162 (11)	0.0181 (4)
O3	0.02430 (16)	0.5769 (3)	0.13491 (11)	0.0171 (4)
O4	0.00354 (17)	0.9410 (3)	0.10079 (12)	0.0214 (4)
O5	0.17839 (17)	0.2632 (3)	0.08359 (12)	0.0185 (4)
O6	0.36985 (17)	0.0892 (3)	0.08995 (12)	0.0215 (4)
O7	0.36109 (17)	0.4588 (3)	0.08306 (12)	0.0239 (4)
O8	0.33457 (19)	0.2831 (3)	0.21070 (12)	0.0269 (5)
O9	0.13156 (17)	0.7477 (3)	−0.15455 (12)	0.0180 (4)
O10	0.28536 (19)	0.7924 (3)	−0.02797 (12)	0.0260 (5)
O11	0.32929 (19)	0.8742 (4)	−0.16031 (12)	0.0338 (5)
O12	0.2992 (2)	0.5090 (4)	−0.12561 (15)	0.0402 (6)
O1W	0.1057 (2)	0.2204 (3)	0.23977 (13)	0.0253 (5)
H1A	0.182 (2)	0.229 (5)	0.257 (2)	0.030*
H1B	0.072 (3)	0.183 (5)	0.2804 (17)	0.030*
O2W	0.53224 (19)	0.7845 (3)	0.07628 (14)	0.0236 (5)
H2A	0.585 (3)	0.674 (4)	0.0852 (19)	0.028*
H2B	0.581 (3)	0.892 (4)	0.0890 (18)	0.028*
O3W	0.4291 (2)	0.7647 (3)	0.21509 (13)	0.0257 (5)
H3A	0.508 (2)	0.761 (5)	0.235 (2)	0.031*
H3B	0.396 (3)	0.733 (5)	0.2572 (18)	0.031*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.0121 (2)	0.0114 (2)	0.0142 (2)	−0.00009 (13)	0.00330 (17)	−0.00003 (12)
Fe2	0.0127 (2)	0.0114 (2)	0.0160 (2)	−0.00012 (14)	0.00287 (16)	0.00014 (13)
S1	0.0114 (3)	0.0098 (3)	0.0142 (3)	−0.0001 (2)	0.0033 (3)	0.0002 (2)
S2	0.0119 (3)	0.0113 (3)	0.0190 (3)	−0.0005 (2)	0.0035 (3)	−0.0002 (2)
S3	0.0123 (3)	0.0205 (4)	0.0167 (3)	0.0007 (3)	0.0038 (3)	−0.0019 (2)
O1	0.0134 (9)	0.0209 (10)	0.0174 (9)	−0.0009 (8)	0.0036 (7)	−0.0011 (7)
O2	0.0173 (9)	0.0209 (10)	0.0155 (9)	−0.0034 (8)	0.0024 (7)	0.0002 (8)
O3	0.0214 (10)	0.0116 (9)	0.0200 (9)	−0.0013 (7)	0.0082 (8)	−0.0003 (7)
O4	0.0183 (10)	0.0114 (9)	0.0364 (11)	0.0002 (8)	0.0101 (8)	−0.0017 (8)
O5	0.0119 (10)	0.0232 (11)	0.0205 (10)	0.0005 (7)	0.0036 (8)	0.0006 (7)
O6	0.0212 (10)	0.0105 (9)	0.0351 (11)	0.0012 (8)	0.0113 (9)	0.0005 (8)
O7	0.0248 (11)	0.0122 (10)	0.0382 (11)	−0.0012 (8)	0.0148 (9)	0.0005 (8)
O8	0.0203 (11)	0.0410 (13)	0.0186 (10)	−0.0012 (9)	0.0022 (8)	−0.0025 (9)
O9	0.0134 (10)	0.0238 (11)	0.0169 (9)	0.0017 (7)	0.0038 (8)	0.0006 (7)
O10	0.0208 (11)	0.0385 (12)	0.0179 (9)	−0.0020 (9)	0.0025 (8)	−0.0020 (9)
O11	0.0291 (12)	0.0477 (14)	0.0266 (11)	−0.0160 (11)	0.0106 (9)	−0.0016 (10)
O12	0.0353 (13)	0.0293 (13)	0.0518 (14)	0.0146 (10)	0.0008 (11)	−0.0092 (10)
O1W	0.0177 (11)	0.0403 (13)	0.0172 (9)	0.0001 (9)	0.0026 (8)	0.0063 (8)
O2W	0.0147 (10)	0.0199 (11)	0.0380 (12)	0.0022 (8)	0.0101 (9)	0.0028 (9)
O3W	0.0174 (11)	0.0421 (14)	0.0164 (10)	−0.0014 (9)	0.0013 (9)	0.0031 (8)

Geometric parameters (Å, º)

Fe1—O9^i	1.932 (2)	S2—O6	1.4668 (19)
Fe1—O2^i	1.9814 (19)	S2—O7	1.479 (2)
Fe1—O5	1.984 (2)	S2—O5	1.487 (2)
Fe1—O4^ii	2.0142 (19)	S3—O12	1.450 (2)
Fe1—O1W	2.016 (2)	S3—O11	1.450 (2)
Fe1—O3	2.0553 (19)	S3—O10	1.481 (2)
Fe2—O10	1.941 (2)	S3—O9	1.500 (2)
Fe2—O7	1.993 (2)	O2—Fe1^i	1.9814 (19)
Fe2—O2W	1.997 (2)	O4—Fe1^iii	2.0142 (19)
Fe2—O3W	1.998 (2)	O6—Fe2^ii	2.008 (2)
Fe2—O6^iii	2.008 (2)	O9—Fe1^i	1.932 (2)
Fe2—O1	2.023 (2)	O1W—H1A	0.85 (2)
S1—O2	1.4651 (19)	O1W—H1B	0.87 (2)
S1—O4	1.4665 (19)	O2W—H2A	0.91 (2)
S1—O1	1.473 (2)	O2W—H2B	0.87 (2)
S1—O3	1.4821 (18)	O3W—H3A	0.88 (2)
S2—O8	1.457 (2)	O3W—H3B	0.88 (2)
O9^i—Fe1—O2^i	93.77 (8)	O2—S1—O3	109.51 (11)
O9^i—Fe1—O5	178.31 (8)	O4—S1—O3	107.12 (11)
O2^i—Fe1—O5	87.50 (8)	O1—S1—O3	108.97 (11)
O9^i—Fe1—O4^ii	88.33 (7)	O8—S2—O6	111.98 (12)
O2^i—Fe1—O4^ii	90.58 (8)	O8—S2—O7	111.85 (12)
O5—Fe1—O4^ii	92.76 (7)	O6—S2—O7	105.49 (12)
O9^i—Fe1—O1W	87.80 (9)	O8—S2—O5	109.79 (12)
O2^i—Fe1—O1W	176.82 (8)	O6—S2—O5	109.15 (11)
O5—Fe1—O1W	90.88 (9)	O7—S2—O5	108.44 (11)
O4^ii—Fe1—O1W	92.23 (8)	O12—S3—O11	113.38 (14)
O9^i—Fe1—O3	87.92 (7)	O12—S3—O10	111.72 (13)
O2^i—Fe1—O3	88.11 (7)	O11—S3—O10	107.99 (12)
O5—Fe1—O3	91.02 (7)	O12—S3—O9	107.97 (12)
O4^ii—Fe1—O3	175.94 (8)	O11—S3—O9	108.26 (12)
O1W—Fe1—O3	89.18 (8)	O10—S3—O9	107.31 (12)
O10—Fe2—O7	90.35 (8)	S1—O1—Fe2	136.79 (12)
O10—Fe2—O2W	95.98 (9)	S1—O2—Fe1^i	149.64 (13)
O7—Fe2—O2W	90.96 (8)	S1—O3—Fe1	132.06 (11)
O10—Fe2—O3W	175.78 (9)	S1—O4—Fe1^iii	142.56 (12)
O7—Fe2—O3W	92.01 (8)	S2—O5—Fe1	141.66 (13)
O2W—Fe2—O3W	87.49 (9)	S2—O6—Fe2^ii	139.79 (12)
O10—Fe2—O6^iii	86.49 (8)	S2—O7—Fe2	139.77 (12)
O7—Fe2—O6^iii	174.52 (8)	S3—O9—Fe1^i	138.94 (12)
O2W—Fe2—O6^iii	84.93 (8)	S3—O10—Fe2	154.28 (15)
O3W—Fe2—O6^iii	91.42 (8)	Fe1—O1W—H1A	123 (2)
O10—Fe2—O1	94.59 (9)	Fe1—O1W—H1B	126 (2)
O7—Fe2—O1	93.19 (7)	H1A—O1W—H1B	111 (3)
O2W—Fe2—O1	168.62 (8)	Fe2—O2W—H2A	125 (2)
O3W—Fe2—O1	81.78 (9)	Fe2—O2W—H2B	125 (2)
O6^iii—Fe2—O1	91.52 (7)	H2A—O2W—H2B	102 (3)
O2—S1—O4	111.23 (11)	Fe2—O3W—H3A	121 (2)
O2—S1—O1	109.53 (11)	Fe2—O3W—H3B	131 (2)
O4—S1—O1	110.43 (11)	H3A—O3W—H3B	106 (3)

Symmetry codes: (i) −x, −y+1, −z; (ii) x, y−1, z; (iii) x, y+1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H1A···O8	0.85 (2)	2.05 (3)	2.754 (3)	140 (3)
O1W—H1B···O3^iv	0.87 (2)	2.05 (2)	2.907 (3)	173 (3)
O2W—H2A···O12^v	0.91 (2)	1.76 (2)	2.655 (3)	166 (3)
O2W—H2B···O11^vi	0.87 (2)	2.01 (2)	2.837 (3)	157 (3)
O3W—H3A···O8^vii	0.88 (2)	1.81 (2)	2.677 (3)	171 (3)
O3W—H3B···O11^viii	0.88 (2)	1.81 (2)	2.676 (3)	171 (3)

Symmetry codes: (iv) −x, y−1/2, −z+1/2; (v) −x+1, −y+1, −z; (vi) −x+1, −y+2, −z; (vii) −x+1, y+1/2, −z+1/2; (viii) x, −y+3/2, z+1/2.