Barlowite, Cu₄FBr(OH)₆, a new mineral isotructural with claringbullite: description and crystal structure

Abstract

The new mineral species barlowite, ideally Cu4FBr(OH)6, has been found at the Great Australia mine, Cloncurry, Queensland, Australia. It is the Br and F analogue of claringbullite. Barlowite forms thin blue, platy, hexagonal crystals up to 0.5 mm wide in a cuprite-quartz-goethite matrix associated with gerhardtite and brochantite. Crystals are transparent to translucent with a vitreous lustre. The streak is sky blue. The Mohs hardness is 2–2.5. The tenacity is brittle, the fracture is irregular and there is one perfect cleavage on {001}. Density could not be measured; the mineral sinks in the heaviest liquid available, diluted Clerici solution (D &3.8 g/cm3). The density calculated from the empirical formula is 4.21 g/cm3. Crystals are readily soluble in cold dilute HCl. The mineral is optically non-pleochroic and uniaxial (–). The following optical constants measured in white light vary slightly suggesting a small variation in the proportions of F, Cl and Br: ω 1.840(4)–1.845(4) and ε 1.833(4)–1.840(4). The empirical formula, calculated on the basis of 18 oxygen atoms and H2O calculated to achieve 8 anions and charge balance, is Cu4.00F1.11Br0.95Cl0.09(OH)5.85. Barlowite is hexagonal, space group P63/mmc, a = 6.6786(2), c = 9.2744(3) Å , V = 358.251(19) Å3, Z = 2. The five strongest lines in the powder X-ray diffraction pattern are [d(Å )(I)(hkl)]: 5.790(100)(010); 2.889(40)(020); 2.707(55)(112); 2.452(40)(022); 1.668(30)(220).

Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states

“…While Zn 2+ does not mix onto the highly Jahn-Teller distorted Cu 2+ kagome sites, up to 15% Cu 2+ can mix onto the pseudooctahedral interlayer Zn 2+ sites, (24) which complicates the observations of intrinsic QSL behavior. (25) Barlowite (26) (Cu 4 (OH) 6 FBr) is another recently discovered mineral with layered Cu 2+ kagome planes that are stacked differently than in herbertsmithite. The Cu 2+ moments between the kagome layers cause long-range magnetic order at T N ≈ 10 − 15 K, much lower than the Curie-Weiss temperature.…”

Synthesis-dependent properties of barlowite and Zn-substituted barlowite

“…The physics of herbertsmithite has been studied ex-Email address: rsmaha@stanford.edu (Rebecca W. Smaha) tensively, but chemical and synthetic limitations have held it back: a small fraction of excess Cu 2+ impurities on the interlayer Zn site results in interlayer magnetic coupling that obscures the intrinsic QSL behavior. [11,12] The mineral barlowite [13,14], Cu 4 (OH) 6 FBr, is another rare example of a material that has an isolated, undistorted S = 1/2 kagomé lattice. It contains Cu 2+ ions on its interlayer site, presumably causing it to have a transition to long-range magnetic order at 15 K. [14,15] Barlowite, therefore, is not a QSL material; however, DFT calculations show that substituting the interlayer site with nonmagnetic Zn 2+ or Mg 2+ should suppress the long-range magnetic order and lead to a QSL state.…”

“…It has been predicted that these differences will yield a significantly lower amount of Cu 2+ impurities on the interlayer site in Zn-or Mg-substituted barlowite compared to herbertsmithite, opening up new avenues to study the intrinsic physics of QSL materials. [17] Here, we re-examine the synthesis of barlowite after noting a discrepancy between the morphology of crystals of natural barlowite (described as "platy" along the caxis [13]) and crystals of synthetic barlowite (rods down the c-axis). [18,19] We present a new method of synthesizing large single crystals of barlowite that are structurally and spectroscopically identical to polycrystalline barlowite at room temperature.…”

Selectively doping barlowite for quantum spin liquid: A first-principles study

“…Very recently, Barlowite Cu 4 (OH) 6 F Br as a new kagomé compound was discovered 19 . Its structure closely resembles Herbertsmithite, whereas the Cu/Zn antisite disorder is automatically avoided.…”