Rezeda M. Sheveleva

Hydrocalumite, a natural Ca-Al Layered Double Hydroxide (LDH), also known as a hydrated calcium aluminate and an aluminate ferrite monosubstituted (AFm) phase in cement chemistry, has been studied by single-crystal and powder X-ray diffraction, electron probe microanalysis and Raman spectroscopy on a sample from Boisséjour, Puy-de-Dôme, Auvergne-Rhône-Alpes, France. The mineral is monoclinic, space group P 2 /c, a = 10.0234(3), b = 11.5131(3), c = 16.2989(5) Å, β = 104.205(3)°, V = 1823.39(9) Å ³ and Z = 4. The crystal structure has been refined to R ₁ = 0.0505 based on 6296 unique reflections. The empirical chemical formula of the mineral (Ca, Al, Cl according to electron probe microanalysis; CO ₂ , OH and H ₂ O on the basis of crystal-structure refinement and Raman spectroscopy) is Ca _3.96 Al _2.04 (OH) _12.04 Cl _0.96 (CO ₃ ) _0.5˙ 5H ₂ O that can be idealized as Ca ₄ Al ₂ (OH) ₁₂ Cl(CO ₃ ) _0.5 ·5H ₂ O. The presence of (CO ₃ ) ^2– and (OH) ^– anionic groups and (H ₂ O) ⁰ molecules is confirmed by Raman spectroscopy. The strongest reflections in the powder X-ray diffraction pattern are ( d in Å, I _rel in %): 7.89, 100; 3.951, 34; 3.860, 59; 3.753, 26; 2.884, 73; 2.527, 27; 2.501, 38; 2.453, 64; 2.433, 30; 2.327, 25. The crystal structure study reveals the ordering of interlayer species: Cl ^– and (CO ₃ ) ^2– anions and H ₂ O molecules, which means that both anions play a species-defining role in contrast to the previous suggestions on mono-anionic end-members (with either Cl or OH or CO ₃ anions). The ordering of two different anions is usually not observed for other LDHs and appears to be a crystal-chemical feature of the Ca-Al LDH members. This work is dedicated to the 150th anniversary of the Mineralogical Society of the UK and Ireland. Hydrocalumite is a mineral with a 90-year history starting from North Ireland, UK, and is a good example of joint efforts of different mineralogical scientific societies in deciphering scientific puzzles.

Conditions of high-temperature volcano-related mineral formation are a source of the new and rare minerals and their associations; they are rather fragmentarily described for volcanic systems as a whole, except for several objects characterized in this regard. The study aim is to present the first results of the mineralogical study of atypical suprasubduction zone neoformation encountered from the Taketomi flank eruption (1933–1934) of the Alaid volcano (Kuril Islands), which has been studied through electron microprobe analyses and powder and single-crystal X-ray diffraction. The following mineral paragenesis is described: diopside, andradite, anorthite, wollastonite, esseneite, wadalite, rhönite-like mineral, fluorite, calcite, apatite, and atacamite. The parageneses of calcium silicates found in volcanic systems are usually interpreted as reworked crustal xenoliths and commonly associated with volcanoes that have a carbonate basement. However, carbonates have not been previously described at the base of the Alaid volcano. Even though the skarn nature of such a mineral paragenesis is possible, we suggest the important role of high-temperature volcanic gases along with the pyrometamorphic effect in the mineral-forming process at depth or in near-surface conditions (fumarole-like type in the form of a system of cracks and burrows). The described mineral paragenesis has not been previously documented, at least for the North Kuril Islands. A detailed mineralogical study of such formations is one of the important steps in understanding the functioning of magmatic systems, the circulation and transformation of natural matter, and mineral-forming processes.

The crystal structure, thermal behavior, and vibrational spectra of the anthropogenic analogue of boussingaultite, (NH₄)₂Mg(SO₄)₂·6H₂O, and its dehydrated counterpart efremovite, (NH₄)₂Mg₂(SO₄)₃, were studied in detail. The sample from the Chelyabinsk burning coal dumps has the composition of (NH₄)_1.92(Mg_1.02Mn_0.01Fe_0.01)_∑1.04(SO₄)₂·6H₂O and crystallizes in the space group P2₁/a, with a = 9.3183(4), b = 12.6070(4), c = 6.2054(3) Å, β = 107.115(5)°, V = 696.70(5) ų (at 20 °C), Z = 2. The thermal evolution steps are as follows: boussingaultite (NH₄)₂Mg(SO₄)₂·6H₂O (25–90 °C) → X-ray amorphous phase (100–150 °C) → efremovite (NH₄)₂Mg₂(SO₄)₃ (160–340 °C) → MgSO₄ Cmcm + Pbnm (340–580 °C) → MgSO₄ Pbnm (580–700 °C). Thermal expansion is anisotropic, with the coefficients (×10⁶ °C⁻¹) α₁₁ = 52(2), α₂₂ = 68(2), α₃₃ = –89(3), and α_v = 31(3) at T = –123 °C; and α₁₁ = 53(2), α₂₂ = 67(2), α₃₃ = 15(1), and α_v = 136(3) at T = 60 °C. The maximal thermal expansion is along the b-axis and is due to straightening of corrugated pseudolayers (within the ab plane) of Mg(H₂O)₆ octahedra and SO₄ tetrahedra with NH₄ groups in the interlayer space. Vibrational spectroscopy outlines the general trend of dehydration and deammonization as the difference in the temperature intervals of these transformation steps allows separation of O–H and N–H vibrations in the process of dehydration by infrared and Raman spectroscopy. The intermediate partially dehydrated modification of boussingaultite was detected by in situ Raman spectroscopy at 110 °C that may correspond to ammonium leonite.

The mineral-like anthropogenic phase Zn(NH₃)₂Cl₂, with ammine (NH₃⁰) complexes from the burned dumps of the Chelyabinsk coal basin (South Urals, Russia), has been investigated using single-crystal and high-temperature powder X-ray diffraction, and Raman and infrared (IR) spectroscopy. The anthropogenic Zn(NH₃)₂Cl₂ is orthorhombic, Imma, a = 7.7399(6), b = 8.0551(5), c = 8.4767(8) Å, V = 528.49(7) ų, R₁ = 0.0388 at −73 °C. Its crystal structure is based upon isolated ZnN₂Cl₂ tetrahedra connected by hydrogen bonds (between NH₃ groups and Cl atoms) into a three-dimensional network. Upon heating, the Zn(NH₃)₂Cl₂ phase is stable up to about 150 °C, which is in good agreement with the data on the temperature of its formation. The crystal structure of Zn(NH₃)Cl₂ expands anisotropically with the strongest thermal expansion observed along the a axis. The thermal expansion of the structure is controlled by the changes in the hydrogen bonding system. The Raman and IR spectroscopic characteristics of this phase are close to those of the mineral ammineite, CuCl₂(NH₃)₂. The studied anthropogenic phase, formed in the unique conditions of burned coal dumps, is identical to the synthetic Zn(NH₃)₂Cl₂.

Alunogen, Al₂(SO₄)₃·17H₂O, occurs as an efflorescent in acid mine drainage, low-temperature fumarolic or pseudofumarolic (such as with coal fires) terrestrial environments. It is considered to be one of the main Al-sulphates of Martian soils, demanding comprehensive crystal-chemical data of natural terrestrial samples. Structural studies of natural alunogen were carried out in the 1970s without localization of H atoms and have not been previously performed for samples from geothermal fields, despite the fact that these environments are considered to be proxies of the Martian conditions. The studied alunogen sample comes from Verkhne–Koshelevsky geothermal field (Koshelev volcano, Kamchatka, Russia). Its chemical formula is somewhat dehydrated, Al₂(SO₄)₃·15.8H₂O. The crystal structure was solved and refined to R₁ = 0.068 based on 5112 unique observed reflections with I > 2σ(I). Alunogen crystalizes in the P-1 space group, a = 7.4194(3), b = 26.9763(9), c = 6.0549(2) Å, α = 90.043(3), β = 97.703(3), γ = 91.673(3) °, V = 1200.41(7) ų, Z = 2. The crystal structure consists of isolated SO₄ tetrahedra, Al(H₂O)₆ octahedra and H₂O molecules connected by hydrogen bonds. The structure refinement includes Al, S and O positions that are similar to previous structure determinations and thirty-four H positions localized for the natural sample first. The study also shows the absence of isomorphic substitutions in the composition of alunogen despite the iron-enriched environment of mineral crystallization. The variability of the alunogen crystal structure is reflected in the number of the “zeolite” H₂O molecules and their splitting. The structural complexity of alunogen and its modifications ranges from 333–346 bits/cell for models with non-localized H atoms to 783–828 bits/cell for models with localized H atoms. The higher values correspond to the higher hydration state of alunogen.

Alunogen, Al2(SO4)3·17H2O, occurs as an efflorescent in acid mine drainage, low-temperature fumarolic or pseudofumarolic (at coal fires) terrestrial environments. It is considered as one of the main Al-sulfates of Martian soils demanding comprehensive crystal chemical data of natural terrestrial samples. However, structural studies of natural alunogen were carried out in 1970s without localization of H atoms and have not previously been performed for samples from geothermal fields, despite the fact that these environments are considered as proxies of the Martian conditions. The studied alunogen sample comes from Verkhne-Koshelevsky geothermal field (Koshelev volcano, Kamchatka, Russia). Its chemical formula is somewhat dehydrated, Al2(SO4)3×15.8 H2O. The crystal structure was solved and refined to R1 = 0.068 based on 5112 unique observed reflections with I > 2σ(I). Alunogen crystalizes in P-1 space group, a = 7.4194(3), b = 26.9763(9), c = 6.0549(2) Å, α = 90.043(3), β = 97.703(3), γ = 91.673(3) °, V = 1200.41(7) Å3, Z = 2. The crystal structure consists of isolated SO4 tetrahedra, Al(H2O)6 octahedra and H2O molecules connected by hydrogen bonds. The structure refinement includes Al, S and O positions that are similar to previous structure determinations and thirty-four H positions localized for the natural sample first. The study also shows the absence of isomorphic substitutions in the composition of alunogen despite the iron-enriched environment of mineral crystallization. The variability of the alunogen crystal structure is reflected in the number of “zeolite” H2O molecules and their splitting. The structure complexity of alunogen and its modifications ranges from 333-346 bits/cell for models with non-localized H atoms to 783-828 bits/cell for models with localized H atoms. The higher values correspond to higher hydration state of alunogen.

The crystal structure of quintinite, Mg4Al2(OH)12(CO3)·3H2O, from Jacupiranga alkaline complex (Cajati, São Paulo, Brazil) has been refined for two samples (91002 and C7029) using single-crystal X-ray diffraction data. The mineral crystallizes in the P-3c1 space group, a = 5.246/5.298, c = 15.110/15.199 Å for the samples 91002/C7029. The crystal structure consists of octahedral sheets with Mg and Al ordering according to the 3 × 3 superstructure. The Mg and Al atoms are coordinated by six hydroxylated oxygen atoms each with the average and bond distances being in the ranges 2.022–2.053 Å and 1.974–1.978 Å, respectively. The interlayer structures are identical (in contradiction to the previous assumptions) and compose from disordered (CO3)2- groups and (H2O)0 molecules. The samples from Jacupiranga can be identified as quintinite-2T that is the second find of the polytype after the Kovdor alkaline complex (Kola peninsula, Russia). The powder X-ray diffraction pattern of quintinite-2T contains weak superstructure reflection at 4.57 Å (010) indicative of Mg and Al ordering. An important crystal-chemical criterion of quintinite is the interlayer distance (d00n-value) of ~ 7.56 Å, which is steady for natural specimens from various findings worldwide.

Halotrichite has been identified in geothermal fields of the Kambalny-Pauzhetka-Koshelev region, the Bolshoi Semiachik complex, and the Mutnovsky volcano (Kamchatka, Russia). Halotrichite forms efflorescence on soils heated up to 70°C, around steam-gas vents and boiling springs. Tschermigite, rozenite, szomolnokite, gypsum, alunogen, barite, melanterite, hexahydrite, and minerals of the copiapite, alunite and voltaite groups were found in close association with halotrichite. The chemical composition of halotrichite samples from different geothermal fields is similar and characterized by Mg substitution in the Fe2+ position, with Fe2+:Mg ratio ranging from 90:10 to 50:50, and also by Fe3+ substitution in the Al position in some samples, with Al:Fe3+ reaching up to 85:15. Halotrichite is a typical mineral of low-temperature volcanic setting, formed as a result of alteration of parent minerals by hydrothermal fluid and represents an intermediate form of crystallization of leached elements. At the same time, the local conditions of mineral formation — such as variations in Eh, pH, surface temperature, and elemental composition — do not reflect in the chemical composition or other thin typomorphic features of halotrichite. The mechanism of halotrichite formation is probably identical in both geothermal fields and zones of oxidation of sulfide deposits.