Synopses: Hohmannite is a hydrated sulphate of ferric iron with the formula Fe₂(SO₄)₂(OH)₂.7H₂O (Palache, Berman, and Frondel, 1951).

Figs. 1 and 2 illustrate the structure of hohmannite. The first shows a complex chain of Fe(O, H₂O)₆ octahedra and SO₄ tetrahedra, which runs along the c axis; the second visualizes the water molecules and an hypothetical hydrogen-bonds system obtained on the basis of electrostatic and geometrical considerations.

Except for the hydroxyl groups, the structure results agree with the composition mentioned above. In fact, according to the hydrogen bonds system shown in fig. 2, no hydroxyl group exists, consequently the chemical formula Fe₂(H₂O)₄ [(SO₄)₂O].4H₂O seems more reliable.

In hohmannite there are two Fe(O, H₂O)₆ octahedra, two SO₄ tetrahedra, four coordinating and four structural waters crystallographically independent. Both Fe(1) and Fe(2) exibite a distorted octahedral coordination with cation-anion distances ranging from 1·93 to 2·06 Å and 1·87 to 2·10 Å respectively. Fe(1) is surrounded by five oxygens and one water molecule, Fe(2) by three oxygens and three waters. The two SO₄ groups have both three longer and one shorter distances. Two centrosymmetrical pairs of Fe(O, H₂O)₆ octahedra and SO₄ tetrahedra are linked together to form a group of composition [Fe₄(H₂O)₄O₈ (SO₄)₄]¹²⁻. These groups polymerize via O(8) to form chains of Fe-O-S linkages along c. Coordinating and structural water molecules provide the hydrogen bond system to connect these chains.

Taking into account the linkages between Fe³⁺(O, OH, H₂O)₆ octahedra and SO₄ tetrahedra Süsse (1971) gives a crystal-chemical classification of some natural ferrisulphates. According to this classification hohmannite, like amarantite, belongs to the second type of the three quoted, i.e. infinite chains of Fe-O-S linkages.

Hohmannite, Fe₂(H₂O)₄[(SO₄)₂O].4H₂O, is in effect a higher hydrate of amarantite, Fe₂(H₂O)₄ [(SO₄)₂O].3H₂O, and has been obtained from amarantite by a partial dehydration followed by a successive rehydration (Césbron, 1964). The solution of the structure of hohmannite permits a useful comparison with the structure of amarantite (Süsse, 1968; Giacovazzo and Menchetti, 1969). Both these minerals have the same P1¯ space group, comparable reticular parameters, and differ chemically only by the water content. This last difference affects the orientation of the chains' repeat unit [Fe₄(H₂O₄)O₈(SO₄)₄]¹²⁻ and the hydrogen bond system. In fact owing to the greater number of water molccules in hohmannite, these units under-go some modification, of which the more important is a rotation of about 50°. The consequence of this is the breakage of the hydrogen bond system of amarantite and the building of a new one in hohmannite.

Scharizer (1927) and Césbron (1964) give for hohmannite and amarantite comparable TGA curves, in agreement with the structural results. The only difference in these curves is that hohmannite starts dehydration at normal temperature, amarantite from 60 °C onwards. The structural explanation is that O(17)w forms the weaker hydrogen bonds and, of course, has the higher temperature factor. So this water seems to be the first to be lost by hohmannite in the reaction amarantite + 1H₂O ⇌ hohmannite.

The structure of hohmannite accounts for some physical properties, as a higher refractive index compared with amarantite, the elongation on the [001] direction and cleavage on {010}, {11¯0}, and {110} quoted in Dana's System of Mineralogy and on {100} (not quoted).