Redetermination of tamarugite, NaAl(SO4)2·6H2O

The crystal structure of tamarugite [sodium aluminium bis(sulfate) hexahydrate] was redetermined from a single crystal from Mina Alcaparossa, near Cerritos Bayos, southwest of Calama, Chile. In contrast to the previous work [Robinson & Fang (1969 ▶). Am. Mineral. 54, 19–30], all non-H atoms were refined with anisotropic displacement parameters and H-atoms were located by difference Fourier methods and refined from X-ray diffraction data. The structure is built up from nearly regular [Al(H2O)6]3+ octahedra and infinite double-stranded chains [Na(SO4)2]3− that extend parallel to [001]. The Na+ cation has a strongly distorted octahedral coordination by sulfate O atoms [Na—O = 2.2709 (11) – 2.5117 (12) Å], of which five are furnished by the chain-building sulfate group S2O4 and one by the non-bridging sulfate group S1O4. The [Na(SO4)2]3− chain features an unusual centrosymmetric group formed by two NaO6 octahedra and two S2O4 tetrahedra sharing five adjacent edges, one between two NaO6 octahedra and two each between the resulting double octahedron and two S2O4 tetrahedra. These groups are then linked into a double-stranded chain via corner-sharing between NaO6 octahedra and S2O4 tetrahedra. The S1O4 group, attached to Na in the terminal position, completes the chains. The [Al(H2O)6]3+ octahedron (〈Al—O〉 = 1.885 (11) Å) donates 12 comparatively strong hydrogen bonds (O⋯O = 2.6665 (14) – 2.7971 (15) Å) to the sulfate O atoms of three neighbouring [Na(SO4)2]3− chains, helping to connect them in three dimensions, but with a prevalence parallel to (010), the cleavage plane of the mineral. Compared with the previous work on tamarugite, the bond precision of Al—O bond lengths as an example improved from 0.024 to 0.001 Å.

The crystal structure of tamarugite [sodium aluminium bis(sulfate) hexahydrate] was redetermined from a single crystal from Mina Alcaparossa, near Cerritos Bayos, southwest of Calama, Chile. In contrast to the previous work [Robinson & Fang (1969). Am. Mineral. 54,[19][20][21][22][23][24][25][26][27][28][29][30], all non-H atoms were refined with anisotropic displacement parameters and H-atoms were located by difference Fourier methods and refined from X-ray diffraction data. The structure is built up from nearly regular [Al(H 2 O) 6 ] 3+ octahedra and infinite double-stranded chains [Na(SO 4 ) 2 ] 3À that extend parallel to [001]. The Na + cation has a strongly distorted octahedral coordination by sulfate O atoms [Na-O = 2.2709 (11) -2.5117 (12) Å ], of which five are furnished by the chainbuilding sulfate group S2O 4 and one by the non-bridging sulfate group S1O 4 . The [Na(SO 4 ) 2 ] 3À chain features an unusual centrosymmetric group formed by two NaO 6 octahedra and two S2O 4 tetrahedra sharing five adjacent edges, one between two NaO 6 octahedra and two each between the resulting double octahedron and two S2O 4 tetrahedra. These groups are then linked into a double-stranded chain via corner-sharing between NaO 6 octahedra and S2O 4 tetrahedra. The S1O4 group, attached to Na in the terminal position, completes the chains. The [Al(H 2 O) 6 ] 3+ octahedron (hAl-Oi = 1.885 (11) Å ) donates 12 comparatively strong hydrogen bonds (OÁ Á ÁO = 2.6665 (14) -2.7971 (15) Å ) to the sulfate O atoms of three neighbouring [Na(SO 4 ) 2 ] 3À chains, helping to connect them in three dimensions, but with a prevalence parallel to (010), the cleavage plane of the mineral. Compared with the previous work on tamarugite, the bond precision of Al-O bond lengths as an example improved from 0.024 to 0.001 Å .
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: PJ2005).

Comment
Tamarugite, NaAl(SO 4 ) 2 ·6H 2 O, is a secondary sulfate mineral which has been found in acidic environments generated by oxidation of sulfides like pyrite in the presence of alkali-rich aluminous rocks as Na and Al source. Classic occurrences of tamarugite are sulfate-rich weathering zones of sulfide ore deposits in the Atacama desert, Chile (Bandy, 1938). Other occurrences concern fumaroles, acid mine drainage, and burning coal dumps (Anthony et al., 2003). The mineral was first described from an occurrence in the Pampa del Tamarugal, Chile, from where it inherited its name (Anthony et al., 2003). The crystal structure of tamarugite was reported by Robinson & Fang (1969) as part of studies on the structural chemistry of salt hydrate minerals of Al 3+ and Fe 3+ . Using diffraction data measured with a Buerger automated diffractometer (Weissenberg geometry, Cu Kα radiation), they obtained R[F] = 0.073 on 744 F hkl with isotropic displacement parameters. They stated that hydrogen atom positions derived from stereochemical considerations were included in this refinement, but did neither report their coordinates nor corresponding geometric parameters.
The present structure redetermination was initiated when during an examination of sulfate mineral specimens from "Alcaparossa" (Mina Alcaparrosa near Cerritos Bayos, southwest of Calama, Chile; see Bandy, 1938) colourless crystals of good quality were encountered that turned out to be tamarugite. Unit cell setting and atom positions reported by Robinson & Fang (1969) were maintained in the present study. A comparison of previous (Robinson & Fang, 1969) and present structural data of tamarugite showed a fair agreement after taking into account that e.s.d.s for atomic coordinates were previously ca 20 times bigger than now, where standard deviations of the Na,Al,S-O bond lengths are about 0.001 Å. The largest difference between the two structures was 0.044 Å for the bond S2-O5 (Table 1). The differences between previous and present non-hydrogen atom positions are 0.018 Å on average and 0.054 Å for O5.
The crystal structure of tamarugite is built up from nearly regular [Al(H 2 O) 6 ] 3+ octahedra and an infinite two-strand chain of the composition [Na(SO 4 ) 2 ] 3extending along [001] (Fig. 1). Na is coordinated by six sulfate oxygen atoms with Na-O distances between 2.2709 (11) and 2.5117 (12) Å, mean value 2.42 Å (σ = 0.10 Å). Five of these six Na-O bonds are to the sulfate group S2O 4 and only Na-O bond to the sulfate group S1O 4 . The coordination figure about Na can be described as a strongly distorted octahedron with cis bond angles between 57.26 (3) and 116.48 (4), and with trans bond angles between 135.96 (4) and 161.83 (5)°. This distortion is mostly due to the presence of a compact centrosymmetric group of two NaO 6 octahedra and two S2O 4 groups joined via five edge-sharing polyhedral links, one between two NaO 6 octahedra and four between these two NaO 6 octahedra and two adjacent SO 4 tetrahedra (Fig. 1). Two corner-sharing links between NaO 6 and S2O 4 polyhedra via O6 expand this group into a two-stranded chain, to which at terminal position the S1O 4 tetrahedron is attached via O3. The two independent sulfate groups S1O 4 and S2O 4 form relatively regular tetrahedra with S-O bond length in the range 1.4653 (10) (Fang & Robinson, 1972;Cromer et al., 1967;Menchetti & Sabelli, 1974, 1976. It is fairly regular by having cis bond angles of 85.98 (5) (Table 2). Eleven hydrogen bonds are largely linear having H···O = 1.78 -2.00 Å and O-H···O = 156 -178° (Table 2). Only the hydrogen bond O14W-H14A···O5 viii is strongly bent due to the arrangement of the acceptor oxgen atom. It has therefore an outlying geometry with H···O = 2.18 Å and O-H···O = 129°. The next nearest oxygen neighbour of H14A, O3 iii with H14A···O3 iii = 2.65 Å, is not regarded as significantly bonded and therefore was not included in Table 2. The packing diagrams shown in Figs. 2 and 3 include all hydrogen bonds of the structure. Fig. 2 shows that each [Al(H 2 O) 6 ] 3+ is hydrogen bonded with three [Na(SO 4 ) 2 ] 3chains. Ten of the twelve different hydrogen bonds help to establish layers parallel to (010) (010) (Anthony et al., 2003). For structural relationships between tamarugite (NaAl(SO 4 ) 2 ·6H 2 O), mendozite (NaAl(SO 4 ) 2 .11H 2 O; contains trans-Na(H 2 O) 4 (SO 4 ) 2 groups and Al(H 2 O) 6 octahedra), and sodium alum (NaAl(SO 4 ) 2 .12H 2 O; contains Na(H 2 O) 6 and Al(H 2 O) 6 octahedra), the reader is referred to Fang & Robinson (1972).

Experimental
Tamarugite used in this study was on a specimen of copiapite and pickeringite from "Alcaparossa, Chile", this is Mina Alcaparrosa near Cerritos Bayos, southwest of Calama, Chile, a location that furnished many well crystallized Fe 3+ sulfate hydrates (Bandy, 1938) and is type locality for the minerals paracoquimbite, parabutlerite and the new species alcaparrosite (K 3 Ti 4+ Fe 3+ (SO 4 ) 4 O(H 2 O) 2 ; Kampf et al., 2012).

Refinement
All hydrogen atoms were clearly visible in a difference Fourier synthesis and refined satisfactorily without restraints. For the final calculations all water molecules were idealized to have O-H = 0.80 Å and H-O-H = 108.0° and were subsequently refined as rigid groups using AFIX 6 of program SHELXL97 (Sheldrick, 2008) with U iso (H) unrestrained.
This refinement method may be considered as an approach to describe the electron density distribution of a water molecule as a fixed aspheric entity that may optionally include idealized nuclear H positions for subsequent geometric calculations.

Computing details
Data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2012); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2010).    The crystal structure of tamarugite in a projection along [100]. Hydrogen bonds are shown as blue lines. Only the atoms of the asymmetric unit are labeled. 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq Na 0.61674 (8)