Astrophyllite is a very rare mineral, a hydrated potassium iron titanium silicate. It is triclinic and occurs as bladed lamellar crystal aggregates or as single elongate crystals in honey-brown to yellow colours. It is found in felsic igneous rocks in fissures and cavities. It may be classed as an inosilicate, phyllosilicate, or an intermediate between the two. This sample, from Cheyenne Canyon in Colorado, USA, 150 mm across.