Atomic Arrangement, Hydrogen Bonding and Structural Complexity of Alunogen, Al 2 (SO 4 ) 3 ·17H 2 O, from Kamchatka Geothermal Field, Russia

• Published in: Crystals
• Main Authors: Elena S. Zhitova, Rezeda M. Sheveleva, Andrey A. Zolotarev, Anton A. Nuzhdaev
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI AG 2023
• Subjects: alunogen; sulphate; geothermal field; Kamchatka; hydrated; crystal structure; Crystallography; QD901-999
• Online Access: https://doi.org/10.3390/cryst13060963; https://doaj.org/article/7ddd14854e254a658b7274c789148fc9

Alunogen, Al 2 (SO 4 ) 3 ·17H 2 O, 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, Al 2 (SO 4 ) 3 ·15.8H 2 O. The crystal structure was solved and refined to R 1 = 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) Å 3 , Z = 2. The crystal structure consists of isolated SO 4 tetrahedra, Al(H 2 O) 6 octahedra and H 2 O 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” H 2 O 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.