The crystal structure of astrophyllite K2Na(Fe, Mn, Mg, rectangle)(7)[Ti-2(
Si4O12)(2)/O-3](OH, F)(4) has been refined. The dimensions of the triclinic
unit cell are: a = 0.5359(2) nm, b = 1.1614(4) nm, c = 1.1861(4) nm, alpha
= 113.16(2)degrees, beta = 103.04(2)degrees, gamma= 94.56(2)degrees, V = 0
.6495(5) nm(3), Z = 1, space group P1, R = 0.057 for 5308 reflections /F-o/
>3 sigma /F-o/. According to structural and compositional differences the m
onoclinic astrophyllite K2NaNa(Fe, Mn)(4)Mg2Ti2[Si4O12](OH)(4)(OH, F)(2) an
d astrophyllite should be considered as two different mineral species. Astr
ophyllite, monoclinic astrophyllite, bafertisite and lamprophyllite contain
heteropolyhedral sheets which topologically are related with Si, O sheets
of mica where one or several SiO4 tetrahedra are replaced by TiOn polyhedra
. Therefore this heterophyliotitanosilicate series represents a kind of fun
ctional substitution in inorganic crystals.