Clinoamphibole lamellae in diopside of garnet lherzolite from Alpe Arami,Bellinzona, Switzerland

Porphyroclastic diopside in garnet lherzolite from Alpe Arami, Bellinzona, Switzerland includes optically-visible clinoamphibole lamellae with a composition intermediate between pargasite and edenite. X-ray and electron microscopic observations show that the diopside crystal contains sub-microscopic thin clinoamphibole lamellae parallel to (010), which have coherent interfaces to the host. A kind of planar defect parallel to (010) in clinopyroxene structure, as suggested by Chisholm (1973), is shown here to correspond to intercalation of a 9 Å lattice fringe of double-chain structure in the electron micrograph of the diopside. The thin clinoamphibole lamellae are observed to be segregated domains consisting of two and more 9 Å fringes.From the chemical characteristics and textural relations of the development of such clinoamphiboles, the chemical change required to form them is considered to have been caused principally by decreasing solubility of atoms such as Na, Al and Cr in clinopyroxene structure during the retrogressive reequilibration. Also, a possibility of finite solid solution of clinoamphibole in clinopyroxene is discussed.