Astrophyllite [(K,Na)3(Fe++,Mn)7Ti2Si8O24(O,OH)7] is an uncommon mineral found mostly in metasomatically altered igneous rocks. It's interesting because it has a neat btanched chain structure. In thin section it resembles stilpnomelane.
Above, view down the c-axis, showing the branched chains. The chains can either be considered pyroxene chains with side branches, or incomplete amphibole chains (as shown by the two outlined tetrahedra. Note that the chains are too closely spaced to connect like a phyllosilicate sheet.
Above: view down the c-axis. Silica tetrahedra are purple with the rear layer lighter in color. Iron octahedra are orange, Sodium coordination polyhedra are yellow and potassium atoms are green. The sodium is in tenfold soordination. The polyhedron can be called an augmented square prism, that is a square prism with a pyramid built onto the end faces. The prism faces are actually slightly non-planar. Titanium octahedra are blue. These fill the gaps between the silica chains.
Above: view down the b-axis. Layers of iron octahedra alternate with silica-layers and alkali metal ions. Titanium octahedra link the silica layers.
Created 22 April 2013, Last Update 29 June 2013