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.

Hydrobasaluminite and basaluminite, two hydrated basic aluminium sulphate minerals have been found in the weathering zone of the Oxford Clay at Crook Hill Brickyard, Chickerell, near Weymouth, Dorset. Hydrobasaluminite occurs as a reaction rim surrounding carbonate concretions, and is believed to have resulted from the neutralization of aluminium-bearing acid sulphate solutions formed by oxidation of pyrite and subsequent leaching of clay. Basaluminite is found only on concretions that have fallen to the floor of the pit, suggesting that it is formed as a dehydration product of hydrobasaluminite.

Chemical analysis of hydrobasaluminite yields the composition 2Al2O3 · SO3 · 20H2O, although this almost certainly includes substantial amounts of adsorbed water. Chemical analysis of basaluminite gives the composition 2Al2O3 · SO3 · 9H2O, which is equivalent to a formula of Al4SO4(OH)10 · 4H2O if it assumed that water is present only as water molecules or hydroxyl ions. The sulphate ions are readily exchangeable. Electron-optical and X-ray powder diffraction data show the minerals to be monoclinic rather than hexagonal as previously reported. Indexed X-ray powder patterns give unit-cell parameters of a = 14.911(5) Å, b = 9.993(2) Å, c = 13.640(5) Å, β = 112.40(4)° for hydrobasaluminite and a = 14.857(3) Å, b = 10.011(3) Å, c = 11.086(7) Å, β = 122.28(3)° for basaluminite. The specific gravity of basaluminite is found to be 2.10 and Z = 4.

Hydrobasaluminite dehydrates irreversibly to basaluminite under normal laboratory conditions, but can be preserved indefinitely at high relative humidity. A study of the dehydration of basaluminite using a diffractometer heating-stage shows the presence of three further distinct hydration states as well as interstratified intermediates. The dehydrations occur topotactically and involve major changes in the c* direction only. DTA and TGA curves can be interpreted in terms of progressive dehydration.

It is suggested that the minerals possess a layer structure, probably containing gibbsite-like double-hydroxide layers with interlayer sulphate ions and water molecules. The data also seem to show a close structural relationship between basaluminite and the hydrated basic aluminium carbonate mineral, scarbroite.

Zektzerite and its isotypes with Zr4+ replaced by Ti4+ or Sn4+ are readily synthesized by fusion of the constituent oxides at 1550 °C followed by recrystallization of the melt or quenched glass at 750–850 °C and I bar pressure. Indexed powder X-ray data for synthetic zektzerites are presented in miniprint Table I. The powder patterns are consistent with an A-centred orthorhombic symmetry. Refined cell dimensions are given in Table III.

The Ti, Zr, and Sn zektzerites melt incongruently. Table IV records melting-point data. The coefficients of thermal expansion of Zr and Ti zektzerite have been determined by X-ray dilatometry. Both have moderately low coefficients of expansion: numerical values are given in miniprint Table II.

Zektzerites contain a six-repeat unit doublechain silicate anion. Although they are chemically similar to milarites, the latter contain double-ring Si12O30 anions. Despite the difference in anion constitution both have structures in which many chemically similar atoms occupy similar positions. The effect of this similarity on the physical properties (e.g. X-ray powder pattern) is enhanced by the similar size and shape of their orthorhombic unit cells. The presence of vacant sites in zektzerite that are potentially available for cation occupancy suggests that chemically complex substitutions may occur; for example LiNa2ScSi6O15 has been synthesized and is believed to be isostructural with zektzerite and emeleusite, LiNa2FeSi6O15.

Colourless, purple, and pink topaz and transparent quartz crystals have been found in calcite veins (with or without milky quartz) in calcareous rocks near Katlang (34° 24′ N, 72° 6′ E), Mardan district. The veins do not contain any fluorite nor the country rocks any topaz. The topaz and transparent quartz crystals are mostly broken and perfectly euhedral outlines are very rare. Refractive indices, 2V, specific gravity, and the fluorine (determined) and H2O + (calculated) contents of two topaz crystals are suggestive of their high 100 OH/(OH+F) ratios (> 25). Rather than being derived, the topaz may have formed in situ by hydrothermal/pneumatolytic activity, followed by tectonic movements that fractured the crystals and resulted in their incorporation in later-formed vein calcite.