Occurrence

Flaggite occurs at the Grand Central mine (coordinates: 31.70250, –110.06194) in the Tombstone district, Cochise County, Arizona, USA, ~1 km south of the town of Tombstone. The three type specimens were originally collected by Sidney A. Williams and were obtained by one of the authors (BT) from Excalibur Minerals. The Grand Central mine exploits a Ag–Au–Pb–Cu–Zn deposit in which the ore, consisting principally of oxidised Ag- and Au-rich galena, occurs in faulted and fractured portions of a large dyke hosted by the Bisbee Group limestone. A good description of the history, geology and mineralogy of the Tombstone district was provided by Williams (Reference Williams1980). Flaggite occurs in cavities in quartz matrix in association with alunite, backite, cerussite, jarosite and rodalquilarite.

Physical and optical properties

Flaggite crystals are tablets (Fig. 1) up to ~0.5 mm across and up to 0.2 mm thick (Fig. 1). On the cotype specimens, flaggite occurs as thin plates grouped in rosettes up to 0.1 mm in diameter (Fig. 2). Tablets are probably flattened on {010} based on the structure, but crystals are too small and irregular to measure forms. Twinning is ubiquitous, by 180° rotation on [001]. Crystals are lime green to yellow green and transparent with adamantine lustre. The streak is very pale green. No fluorescence was observed in either longwave or shortwave ultraviolet illumination. The Mohs hardness is ~3 based upon scratch tests. Crystals are brittle with irregular fracture. There is one excellent cleavage on {010}. The density could not be measured because it exceeds that of available density liquids and there is an insufficient quantity for physical measurement. The calculated density based on the empirical formula and unit-cell parameters obtained from single-crystal X-ray diffraction data is 6.137 g cm^–3 and that for the ideal formula is 6.212 g cm^–3. At room temperature, flaggite decomposes in dilute HCl, losing birefringence and becoming white and translucent.

Fig. 1.

Flaggite tablets (green) with alunite (yellow) on quartz; holotype specimen #76143; field of view 0.45 mm across.

Fig. 2.

Rosette of flaggite plates (green) with backite (grey) and jarosite (yellow orange) on quartz; cotype specimen #64499; field of view 0.56 mm across.

Optically, flaggite is biaxial (+), with α = 1.95(1), β = 1.96(1) and γ = 2.00(1) measured in white light. The 2V measured using extinction data with EXCALIBR (Gunter et al., Reference Gunter, Bandli, Bloss, Evans, Su and Weaver2004) is 54(2)° and the calculated 2V is 54.0°. The dispersion could not be observed and the optical orientation was not determined because of the difficulty in determining the orientation of the irregularly shaped crystals. The pleochroism is X green, Y light yellow green and Z nearly colourless; X > Y > Z. The Gladstone–Dale compatibility index 1 – (K _P/K _C) for the empirical formula is –0.012 and that for the ideal formula is –0.003, in both cases in the superior range (Mandarino, Reference Mandarino2007).

Raman Spectroscopy

Raman spectroscopy was conducted on a Horiba XploRA PLUS using a 532 nm diode laser, 50 μm slit, 2400 gr/mm diffraction grating and a 100× (0.9 NA) objective. Selected regions of the spectrum recorded perpendicular to the {010} cleavage are shown in Fig. 3. The Raman spectrum is very complex due to the low symmetry of the structure and the large number of independent cation sites. The assignments for the TeO₆ v ₁ symmetric stretching and SO₄ v ₁ symmetric stretching are straightforward and are diagnostic for this mineral, and they have been labelled in Fig. 3, as has the weak band at 1092 cm^–1 and the broad associated features, which are related to SO₄ v ₃ antisymmetric stretching. Bairdite on the other hand has only a single SO₄ v ₁ symmetric stretching band at 977 cm^–1 and the TeO₆ v ₁ symmetric stretching band is located at 721 cm^–1 (Kampf et al., Reference Kampf, Mills, Housley, Rossman, Marty and Thorne2013). We do not feel confident in assigning modes to specific bands in the 650 to 300 cm^–1 range; however, these all are likely to be due to CuO₆ and TeO₆ stretching, as well as SO₄, CuO₆, and TeO₆ bending modes. The very weak band at 1425 cm^–1 could be due to H₂O bending. The spectrum is featureless between the 1425 cm^–1 band and the OH stretching region.

Fig. 3.

Selected regions of the Raman spectrum of flaggite recorded perpendicular to the {010} cleavage with a 532 nm laser. Top left: region for CuO₆ and TeO₆ stretching, as well as SO₄, CuO₆, and TeO₆ bending modes. Left middle: region for SO₄ and TeO₆ stretching modes. Left bottom: region for OH stretching modes. Right: overview of Raman spectrum.

Chemical composition

Analyses (8 points) were performed at Caltech on a JEOL 8200 electron microprobe in WDS mode. Analytical conditions were 15 kV accelerating voltage, 10 nA beam current and 2 μm beam diameter. Insufficient material is available for CHN analysis; however, the fully ordered structure unambiguously established the quantitative content of H₂O. Analytical data are given in Table 1. The empirical formula based on S = 2 and O = 22 atoms per formula unit is Pb_3.88Cu²⁺_3.89Te⁶⁺_2.08(SO₄)₂O₁₁(OH)₂(H₂O) (–0.03 H for charge balance). The ideal formula is Pb₄Cu²⁺₄Te⁶⁺₂(SO₄)₂O₁₁(OH)₂(H₂O), which requires PbO 50.78, CuO 18.10, TeO₃ 19.97, SO₃ 9.11, H₂O 2.05, total 100 wt.%.

Table 1.

Chemical composition (in wt.%) for flaggite.

*Based upon the crystal structure with S = 2 and O = 22 atoms per formula unit.

S.D. – standard deviation

X-ray crystallography and structure refinement

Powder X-ray diffraction was done using a Rigaku R-Axis Rapid II curved-imaging-plate microdiffractometer, with monochromatised MoKα radiation. A Gandolfi-like motion on the φ and ω axes was used to randomiSe the sample and observed d values and intensities were derived by profile fitting using JADE Pro software (Materials Data, Inc.). The powder data are presented in Supplementary Table S1 (see below).

Single-crystal X-ray studies were carried out on a Bruker APEX2 CCD diffractometer equipped with MoKα radiation. The intensity data of flaggite were extracted from a crystal twinned by 180° rotation on [001]. All data were processed using Bruker TWINABS. The crystal structure was solved and refined in space group P1 using SHELX (Sheldrick, Reference Sheldrick2015a, Reference Sheldrick2015b). The refinement using the data from a single component was better than that using both components, so the single-component refinement is reported here. The limited data set, imperfect absorption correction and problems in separating the twinned data compromised the quality of the data resulting in large residual electron densities and problems with anisotropic displacement parameters. Consequently, the non-sulfate O atoms (O17–O44) could only be refined isotropically and several of the Cu, Te and sulfate O atoms, which were refined anisotropically, exhibited markedly prolate or oblate ellipsoids. Data collection and refinement details are given in Table 2, atom coordinates and displacement parameters in Table 3, selected interatomic distances in Table 4 and bond-valence sums (BVS) in Table 5. The crystallographic information file (cif) has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material. Note that the processing of the twinned structure data required merging of equivalent reflections prior running SHELX. Consequently, there is no R _int listed in the deposited cif. The number of reflections before merging and the R _int have been included in Table 2.

Table 2.

Data collection and structure refinement details for flaggite.*

*R _int = Σ|F _o²–F _o²(mean)|/Σ[F _o²]. GoF = S = {Σ[w(F _o²–F _c²)²]/(n–p)}^½. R ₁ = Σ||F _o|–|F _c||/Σ|F _o|. wR ₂ = {Σ[w(F _o²–F _c²)²]/Σ[w(F _o²)²]}^½; w = 1/[σ²(F _o²) + (aP)² + bP] where a is 0.0241, b is 0 and P is [2F _c² + Max(F _o²,0)]/3.

Table 3.

Atom coordinates and displacement parameters (Ų) for flaggite.

Table 4.

Selected interatomic distances (Å) in flaggite.

Table 5.

Bond-valence sums for flaggite*. Values are expressed in valence units.

* Te⁶⁺–O bond valence parameters are from Mills and Christy (Reference Mills and Christy2013). All others are from Gagné and Hawthorne (Reference Gagné and Hawthorne2015). Hydrogen-bond strengths are based on O–O bond lengths from Ferraris and Ivaldi (Reference Ferraris and Ivaldi1988). Negative values indicate donated hydrogen-bond valences.

Description and discussion of the structure

In the structure of flaggite (Fig. 4), individual TeO₆ octahedra and pairs of edge-sharing Jahn-Teller distorted CuO₆ octahedra link by edge-sharing into chains along [001]. The chains are linked to one another by corner-sharing to form stair-step-like layers parallel to {010} (Fig. 5). The region between the layers contains SO₄ tetrahedra and Pb²⁺. The SO₄ tetrahedra link by corner sharing to the apical O atoms of the CuO₆ octahedra projecting from only one side of the octahedral layer. The eight distinct Pb²⁺ atoms bond to either eight or nine O atoms. The Pb²⁺–O bonds cover a broad range (2.274 to 3.451 Å); however, there are no pronounced lopsided distributions of bond lengths typical of Pb²⁺ with stereoactive 6s² lone-pair electrons.

Fig. 4.

The structures of flaggite and bairdite. Pb atoms are blue, SO₄ tetrahedra are red, TeO₆ octahedra are yellow, CuO₆ octahedra are green. The unit cell outlines are shown with thick black lines.

Fig. 5.

Stair-step-like layer of edge-sharing TeO₆ and CuO₆ octahedra in the structure of flaggite. The unit cell outline is shown with thick black lines.

The same types of edge-sharing chains forming stair-step-like octahedral layers occur in the structures of bairdite, Pb₂Cu²⁺₄Te⁶⁺₂O₁₀(OH)₂(SO₄)(H₂O) (Kampf et al., Reference Kampf, Mills, Housley, Rossman, Marty and Thorne2013), timroseite, Pb₂Cu²⁺₅(Te⁶⁺O₆)₂(OH)₂ (Kampf et al., Reference Kampf, Mills, Housley, Marty and Thorne2010), and paratimroseite, Pb₂Cu²⁺₄(Te⁶⁺O₆)₂(H₂O)₂ (Kampf et al., Reference Kampf, Mills, Housley, Marty and Thorne2010). The structure is most similar to that of bairdite; however, there is a double stair-step layer in bairdite, as well as SO₄ groups projecting from both directions into the interlayer (Fig. 4), although it should be noted that the SO₄ groups in the bairdite structure are half-occupied. The flaggite structure can be derived from that of bairdite by removing one stair-step layer and the SO₄ groups projecting in one direction. An interesting feature of the stair-step-like layers in all of these structures is that they are based upon hexagonal close packing (HCP), not only in terms of the individual steps (or chains), but also with respect to the continuous assembly of steps. In the structural classification of Te oxycompounds of Christy et al. (Reference Christy, Mills and Kampf2016b), flaggite has a structure with monomeric Te⁶⁺X ₆ as part of a larger structural unit that is an infinite layer.