Jahn-Teller Distortion and Cation Ordering: The Crystal Structure of Paratooite-(La), a Superstructure of Carbocernaite

Abstract

The crystal structure of paratooite-(La) has been solved using crystals from the type locality, Paratoo copper mine, near Yunta, Olary Province, South Australia, Australia. The mineral is orthorhombic, Pbam, a = 7.2250(3) Å, b = 12.7626(5) Å, c = 10.0559(4) Å, V = 927.25(6) Å 3 , and R 1 = 0.063 for 1299 unique observed reflections. The crystal structure contains eight symmetrically independent cation sites. The La site, which accommodates rare earth elements (REEs), but also contains Sr and Ca, has a tenfold coordination by seven carbonate groups. The Ca, Na1, and Na2 sites are coordinated by eight, eight, and six O atoms, respectively, forming distorted CaO 8 and Na1O 8 cubes, and Na 2 O 6 octahedra. The Cu site is occupied solely by copper and possess a distorted octahedral coordination with four short (1.941 Å) and two longer (2.676 Å) apical Cu-O bonds. There are three symmetrically independent carbonate groups (CO 3 ) 2− with the average <C-O> bond lengths equal to 1.279, 1.280, and 1.279 Å for the C1, C2, and C3 sites, respectively. The crystal structure of paratooite-(La) can be described as a strongly distorted body-centered lattice formed by metal cations with (CO 3 ) 2− groups filling its interstices. According to the chemical and crystal-structure data, the crystal-chemical formula of paratooite-(La) can be described as (La 0.74 Ca 0.11 Sr 0.07 ) 4 CuCa(Na 0.75 Ca 0.15 )(Na 0.63 )(CO 3 ) 8 or REE 2.96 Ca 1.59 Na 1.38 CuSr 0.28 (CO 3 ) 8 . The idealized formula can be written as (La,Sr,Ca) 4 CuCa(Na,Ca) 2 (CO 3 ) 8 . The structure of paratooite is a 1 × 2 × 2 superstructure of carbocernaite, CaSr(CO 3 ) 2 . The superstructure arises due to the ordering of the chemically different Cu 2+ cations, on one hand, and Na + and Ca 2+ cations, on the other hand. The formation of a superstructure due to the cation ordering in paratooite-(La) compared to carbocernaite results in the multiple increase of structural complexity per unit cell. Therefore, paratooite-(La) versus carbocernaite represents a good example of structural complexity increasing due to the increasing chemical complexity controlled by different electronic properties of mineral-forming chemical elements (transitional versus alkali and alkaline earth metals).

Mckelveyite group minerals – Part 4: Alicewilsonite-(YLa), Na2Sr2YLa(CO3)6 ⋅ 3H2O, a new lanthanum-dominant species from the Paratoo mine, Australia

“…That the latter may result in preferential leaching of trivalent REE was demonstrated by Taunton et al (2000) in a granite. The fractionation of Ce 4+ led to the formation of Ladominant secondary REE carbonates, such as paratooite-(La), (La,Sr,Ca) 4 CuCa(Na,Ca) 2 (CO 3 ) 8 (Pring et al, 2006;Krivovichev et al, 2019), bastnäsite-(La), La(CO 3 )F (Pring et al, 2006), and alicewilsonite-(YLa).…”

Structural and chemical complexity of minerals: an update

“…The list of twenty most structurally complex minerals known so far given in Table 3 reveals the following most important complexity-generating mechanisms in minerals: the presence of large (sometimes nanometre-scale) clusters such as polyoxometalates or related finite-cluster structures (Krivovichev, 2020b) isolated from each other; such multinuclear atomic units possess a large number of atoms with different topological functions; the examples are ewingite, morrisonite, vanarsite, bouazzerite and postite; as a rule, natural polyoxometalate minerals are also highly hydrated, with the exception of arsmirandite and lehmannite, which are anhydrous polyoxocuprates formed in volcanic fumaroles (Britvin et al , 2020); the presence of large clusters linked together to form three-dimensional frameworks; the examples are ilmajokite and paddlewheelite (both minerals are also highly hydrated, which contributes greatly to their structural complexities); the formation of complex three-dimensional modular frameworks formed by cages of different sizes and topologies; this complexity type corresponds to paulingite-group minerals, fantappièite and sacrofanite (members of the sodalite–cancrinite ABC series (Bonaccorsi and Nazzareni, 2015; Chukanov et al , 2021), tschörtnerite, mendeleevite-(Ce) and rowleyite; the formation of complex layers with different combinations of modules (chains or rings); this type is characteristic for layered uranyl minerals (such as vandendriesscheite and gauthierite) and layered silicates (parsettensite); a high hydration state in salts with complex heteropolyhedral units (alfredstelznerite and voltaite-group minerals); the formation of ordered superstructures of relatively simple structure types; the examples are meerschautite (which is the only sulfide species in the list and was described by Biagioni et al (2016) as an expanded derivative of owyheeite) and manitobaite (that has a fivefold superstructure relative to alluaudite (Tait et al , 2011)); for further discussion on the relations between superstructures and complexity see Krivovichev et al (2019a, 2021), Kornyakov et al (2021) and Kornyakov and Krivovichev (2021). …”

“…the formation of ordered superstructures of relatively simple structure types; the examples are meerschautite (which is the only sulfide species in the list and was described by Biagioni et al (2016) as an expanded derivative of owyheeite) and manitobaite (that has a fivefold superstructure relative to alluaudite (Tait et al , 2011)); for further discussion on the relations between superstructures and complexity see Krivovichev et al (2019a, 2021), Kornyakov et al (2021) and Kornyakov and Krivovichev (2021).…”