Epsomite
Steven Dutch, Professor Emeritus, Natural and Applied Sciences,
Epsomite(MgSO4•7(H2O)) is one of the more familiar musherals. It consists of magnesium-water octahedra linked by sulfate ions and additional water molecules (six per magnesium atom in each octahedron, plus one extra interstitial). One might expect melanterite (FeSO4•7(H2O)) and boothite (CuSO4•7(H2O)) to have similar structures, but although there is some resemblance, those minerals are monoclinic, . One might also expect hexahydrite (FeSO4•6(H2O)) to have the epsomite structure minus that extra water molecule, but it, too is monoclinic. In the diagram below, sulfate ions are yellow, magnesium-water octahedra are green, and independent water molecules have blue oxygen and pink hydrogen. Objects in the foreground have darker, more bold colors than those in the background. The view is down the c axis with b extending horizontally and a vertically.
Although it's orthorhombic, the symmetry class is 222 (disphenoidal). Making it even harder to see relationships, the two-fold axes are screw axes (The point group is p212121), meaning that features related by symmetry are offset. Also some symmetry axes are not neatly aligned with cell edges and centers. The low symmetry is further obscured by the fact that neighboring octahedra are roughly equidistant but all related by symmetry in different ways. The figure below shows the three sets of screw axes. Red axes are perpendicular to the plane of the diagram. The purple bands are axes in the plane of the diagram (lying along the top and bottom faces of the unit cell). The tan bands are one half unit cell below the plane of the diagram.
The view below is along the a axis, with b horizontal and c vertical. The symmetry is a lot easier to see in this direction. Twofold screw axes in and out of the plane are located between the pairs of octahedra and sulfate ions. Screw axes extend horizontally along the strips of alternating octahedra and tetrahedra. They coincide with unit cell edges (purple) and also run through the centers of the unit cells. Other screw axes run vertically, again coinciding with unit cell edges and the center lines of the cells.
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Created 04 April 2011, Last Update 15 Jan 2020