Structural considerations of some natural and artificial alkali iron hydrated sulphates
Authors: Scordari F. 
Published: December 2006

Synopsis

Two compounds are described which are structurally related to the three salts, metavoltine, α-Maus' salt, and an unnamed compound here termed ‘salt X’: The last three salts have structures based on two sheets of composition [K Fe33+O(SO4)6(H2O)3]4− which are essentially intercon-nected via Na atoms in metavoltine, K-atoms in α-Maus' salt and Na, K, H3O in salt X.

Cell parameters and space groups of the new compounds have been determined: as in metavoltine, α-Maus' salt, and salt X they have a = b ≃ 9.6 Å, but the c parameters are ≃ 36 Å and ≃ 52 Å, i.e. about double and triple that of the other three salts (≃ 18 Å).

The salt with c ≃ 36 Å was obtained using Van Tassel' recipe at about the same conditions which gave rise to salt X, i.e. from a solution containing Na2SO4·10H2O, K2SO4, Fe2(SO4)3·nH2O (n ≃ 7H2O) in the ratio 3.40:0.60:4.30. The salt with c ≃ 52 Å was formed spontaneously through a topotactic reaction of salt X.

It is possible to deduce for these two new salts:

(1) The presence of clusters of composition [Fe33+O(SO4)6(H2O)3]5− interconnected via K atoms to build sheets like those found in meta-voltine, α-Maus' salt, and salt X.

Some sheet sequences which may include the correct sequence.

The position of Fe3+ atoms belonging to clusters, which are connected by K atoms to form sheets.

The similar a and b lattice parameters, as well as the presence of the threefold axis in all the compounds mentioned, suggest strongly that in the two new compounds there are sheets of composition [K Fe33+O(SO4)6(H2O)3]4− which are characterized by a = b ≃ 9.6 Å and threefold symmetry. As for the c parameters, examination of possible stacking sequences of the sheets shows which sequences are consistent with the known c parameters and symmetry. The two salts have a parameter and symmetry more similar to metavoltine and salt X than to α-Maus' salt; moreover, metavoltine and salt X, like the new salts, are more stable than α-Maus' salt. That is why probably in the two new salts there are two sheets interconnected by Na atoms as in metavoltine and salt X to form a sandwich sheet of composition [Na2K2Fe63+O2(SO4)12(H2O)6]6−. Then, taking into account the c parameters and space groups, possible sequences of these sandwich sheets are considered in order to establish the likely sequence in the new salts.

Metavoltine and salt X contain clusters [Fe33+O(SO4)6(H2O)3]5− which are differently oriented: the Fe3+ cation lies at x = 0.17, y = 0.22 or x = 0.22, y = 0.17. These two Fe3+ positions involve an angle rotation for the cluster of about 25°. Owing to the similarity in a, b parameters of metavoltine and the salt with c ≃ 52 Å on one hand, and of salt X and the salt with c ≃ 36 Å on the other, it seems quite probable that the xy coordinates of the Fe3+ cations belonging to the first pair of salts are similar, and similarly for the second pair.

(Na, K) α-Maus' salt effloresces in air, the products being ferrinatrite and goldichite. The transformation (Na, K) α-Maus' salt → ferrinatrite was clarified when the structure of ferrinatrite was solved. The transition (Na, K) α-Maus' salt →, goldichite is considered in order to understand how [Fe33+O(SO4)6(H2O)3]5− clusters may be modified to build new structural units such as the corrugated sheets present in goldichite.