Editorial for Special Issue “New Mineral Species and Their Crystal Structures”

Mineralogy is the oldest and one of the most important sciences of the geological cycle. Minerals, the basis of overwhelming mass of solid matter in the universe, are direct subjects of investigation in mineralogy. Minerals, or mineral species, are generally solid crystalline substances. Their definition indicates that, they are: (1) naturally occurring; (2) belonging to the distinct structural type; (3) stable, varying merely in the relatively small limits of chemical composition. If a given mineral differs from other known species in its structure (2) and/or composition (3) then it can be considered as a new mineral species[…

Raman imaging as a new approach to identification of the mayenite group minerals

The mayenite group includes minerals with common formula Ca12Al14O32−x(OH)3x[W6−3x], where W = F, Cl, OH, H2O and x = 0–2. This distinction in the composition is associated with W site which may remain unoccupied or be occupied by negatively charged ions: OH−, F−, Cl−, as well as neutral molecules like H2O. However, there is no experimental approach to easily detect or differentiate mineral species within the mayenite group. Electron micro-beam facilities with energy- or wavelength-dispersive X-ray detectors, as most common tools in mineralogy, appear to be insufficient and do not provide a definite identification, especially, of hydroxylated or hydrated phases. Some solution provides typical Raman analysis ensuring identification of minerals and 3D Raman imaging as an innovative approach to distinguish various co-existing minerals of the mayenite group within a small area of the rock sample. Raman spectroscopy has also been successfully used for a determination of water type incorporated into the mineral structure as well as for a spatial distribution of phases by cluster approach analysis and/or integrated intensity analysis of bands in the hydroxyl region. In this study, Raman technique was for the first time used to reconstruct a 3D model of mayenite group mineral zonation, as well as to determine a way of water incorporation in the structure of these minerals. Moreover, for the first time, Raman data were correlated with alterations during the mineral-forming processes and used for reconstruction of the thermal history of studied rock. As a result, the influence of combustion gases has been proposed as a crucial factor responsible for the transformation between fluormayenite and fluorkyuygenite

Second occurrence of the new mineral harmunite CaFe2O4, Negev Desert, Israel.

Harmunite (ideally CaFe2O4) was found in the natural environment for the first time in 2014 in pyrometamorpic larnite rocks of the Hatrurim Complex that lies near Jabel Harmun – moutain located in Judean Desert, Israel - from which it derives its name (Galuskina et al. 2014). Macroscopically, together with srebrodolskite and magnesioferrite, harmunite creates black porous aggregates (Galuskina et al. 2014). In reflected light with crossed polars it has light gray colour with characteristic red internal reflections (Galuskina et al. 2014). Harmunite occurs as crystal faceted by the simple forms {100}, {110}, {210}, {011}, {001}, and {010} or as rounded fragments (Galuskina et al. 2014). The structure of CaFe2O4 consist of double rutile-type ∞1[Fe2O6] chains, which are further linked by common oxygen corners creating a tunnel-structure with large trigonal prismatic cavities occupied by Ca along [001] (Galuskina et al. 2014). Synthetic compound CaFe2O4 is known and used as ceramic material and pigment, semiconductors, refractories, thermally stable material and others (Candeia et al. 2004, Kharton et al. 2008). This phase was also previously found in the Salair pyrometamorphic complex of Kuznetsky coal basin in south – west Siberia, Russia (Nigmatulina & Nigmatulina 2009) and Chelabynsk coal basin, Southern Urals, Russia (Chesnokov et al, 1998) and described as “aciculite”, but it was not approved as a mineral due to its anthropogenic origin (Galuskina et al. 2014). We found harmunite in pyrometamorphic gehlenite rocks of the Hatrurim Complex located in north – east part of Negev Desert, Israel. As for the holotype specimen, it forms aggregates with srebrodolskite and Mg – ferrite. Single grains of harmunite from Negev reach about 25 μm in size. In comparison with holotype specimen, this harmunite contains more varied substitution at octahedral site , where Fe3+ is substituted by Cu, Ni or Zn. Futhermore, there is no Al, which was noted in holotype harmunite. The Raman spectrum of harmunite from Negev is similar to spectrum of holotype specimen and of the synthetic analog. The main Raman bands of harmunite from Negev are as follows (cm–1): 1241, 648, 601, 526, 439, 376, 301, 277, 214, 166, 131, 91

Walstromite, BaCa2(Si3O9), from Rankinite Paralava within Gehlenite Hornfels of the Hatrurim Basin, Negev Desert, Israel

Walstromite, BaCa2Si3O9, known only from metamorphic rocks of North America, was found in small veins of unusual rankinite paralava within gehlenite hornfelses of the Hatrurim Complex, Israel. It was detected at two localities—Gurim Anticline and Zuk Tamrur, Hatrurim Basin, Negev Desert. The structure of Israeli walstromite [with P1 space group and cell parameters a = 6.74874(10)Å, b = 9.62922(11) Å, c = 6.69994(12) Å, α = 69.6585(13)°, β = 102.3446(14)°, γ = 96.8782(11)°, Z = 2, V = 398.314(11) Å3) is analogous to the structure of walstromite from type locality—Rush Creek, eastern Fresno County, California, USA. The Raman spectra of all tree minerals exhibit bands related to stretching symmetric vibrations of Si-O-Si at 650–660 cm−1 and Si-O at 960–990 cm−1 in three-membered rings (Si3O9)6−. This new genetic pyrometamorphic type of walstromite forms out of the differentiated melt portions enriched in Ba, V, S, P, U, K, Na, Ti and F, a residuum after crystallization of rock-forming minerals of the paralava (rankinite, gehlenite-åkermanite-alumoåkermanite, schorlomite-andradite series and wollastonite). Walstromite associates with other Ba-minerals, also products of the residual melt crystallization as zadovite, BaCa6[(SiO4)(PO4)](PO4)2F and gurimite, Ba3(VO4)2. The genesis of unusual barium mineralization in rankinite paralava is discussed. Walstromite is isostructural with minerals—margarosanite, BaCa2Si3O9 and breyite, CaCa2(Si3O9), discovered in 2018

Not Only Garnets…

Garnets have been known to man since time immemorial and are used in a wide variety of applications as well as being prototypes of useful synthetic materials. Our investigations show that in nature, garnets and minerals with a langasite-type structure can be very close in composition. Examples are cubic Ti-rich garnets with the common formula Ca3(Ti4+,Fe3+,Al)2(Si,Fe3+,Al)3O12 and the new trigonal mineral qeltite, Ca3Ti(Fe3+2Si)Si2O14, which occur in paralavas of the pyrometamorphic Hatrurim Complex, Israel. Synthetic compounds of the langasite family are important because of their functional properties, such as unique piezoelectricity, high thermal stability, and low acoustic losses, as well as optical nonlinearity and multiferroicity. Qeltite is the first high-temperature terrestrial mineral with a langasite-type structure, the description of which was a catalyst for the discovery in pyrometamorphic rocks of the Hatrurim Complex of a whole series of new natural phases with langasite-type structure and varied composition (A3BC3D2O14, where A = Ca and Ba; B = Ti, Nb, Sb, and Zr; C = Ti, Al, Fe, and Si; and D = Si). We think that qeltite and other minerals with langasite-type structure may be relatively widely distributed in terrestrial rocks that form under similar conditions to those of Ti-rich garnet but are missed by researchers

Molecular Hydrogen in Natural Mayenite

In the last 15 years, zeolite-like mayenite, Ca12Al14O33, has attracted significant attention in material science for its variety of potential applications and for its simple composition. Hydrogen plays a key role in processes of electride material synthesis from pristine mayenite: {Ca12Al14O32}2+(O2)!{Ca12Al14O32}2+(e)2. Apresence of molecular hydrogen in synthetic mayenite was not confirmed by the direct methods. Spectroscopy investigations of mayenite group mineral fluorkyuygenite, with empirical formula (Ca12.09Na0.03)P 12.12(Al13.67Si0.12Fe3+ 0.07Ti4+ 0.01)P 12.87O31.96 [F2.02Cl0.02(H2O)3.22(H2S)0.15 0.59]P 6.00, show the presence of an unusual band at 4038 cm1, registered for the first time and related to molecular hydrogen, apart from usual bands responding to vibrations of mayenite framework. The band at 4038 cm1 corresponding to stretching vibrations of H2 is at lower frequencies in comparison with positions of analogous bands of gaseous H2 (4156 cm1) and H2 adsorbed at active cation sites of zeolites (4050–4100 cm1). This points out relatively strong linking of molecular hydrogen with the fluorkyuygenite framework. An appearance ofH2 in the fluorkyuyginite with ideal formula Ca12Al14O32[(H2O)4F2], which formed after fluormayenite, Ca12Al14O32[ 4F2], is connected with its genesis. Fluorkyuygenite was detected in gehlenite fragments within brecciaed pyrometamorphic rock (Hatrurim Basin, Negev Desert, Israel), which contains reduced mineral assemblage of the Fe-P-C system (native iron, schreibersite, barringerite, murashkoite, and cohenite). The origin of phosphide-bearing associations is connected with the e ect of highly reduced gases on earlier formed pyrometamorphic rocks

Different route of hydroxide incorporation and thermal stability of new type of water clathrate : X-ray single crystal and Raman investigation

Chlormayenite Ca12Al14O32[♦4Cl2] (♦-vacancy) is partially hydrated micro porouss mineral with hydroxide groups situated at various crystallographic sites. There are few mechanisms describing its hydration. The first one assumes Cl- substitution by OH- at the center of the structural cages (W-site). The second one determines the converting a T1O4 tetrahedron to a T1O3(OH)3 octahedron due to the replacement of oxygen at the O2 site by three OH-groups according to the scheme: (O2O2- + W Cl-) → 3 × O2aOH. The third mechanism, not considered so far in the case of zeolite-like minerals, includes the hydroxide incorporation in form of hydrogarnet defect due to the arrangement of tetrahedral (OH)4 in vacant cages. This yields a strong hydrated phase containing even up to 35% of water more than in any currently known mineral applicable to Portland cement. Moreover, water molecules present in different structural cages are stable up to 355 K while dehydroxylation linked to the gradual loss of only 8% of OH- groups according to 3 O2aOH- → O2O2- + W OH- + gH2O occurs at temperature range from 355 K to 598 K