COMPLEX MECHANISMS OF ALTERATION IN A GRAFTONITE + SARCOPSIDE + TRIPHYLITE ASSOCIATION FROM THE LUNA PEGMATITE, PIONA, LECCO PROVINCE, ITALY

Abstract

Phosphate masses have been found in a pegmatite rich in oligoclase, with minor muscovite and schorl, outcropping at Luna, near Piona, Lecco Province, Italy. These masses consist of sarcopside lamellae embedded in graftonite with minor triphylite with a well-developed alteration rim. Chemical analyses made with an electron microprobe and the X-ray powder-diffraction method allowed us to identify the following alteration-induced phosphate minerals: ferrisicklerite, heterosite, kryzhanovskite, ferrostrunzite, jahnsite-(CaMnFe), rockbridgeite, lipscombite, and whitmoreite. Petrographic observations of the textures allowed us to decipher the complex mechanism of alteration. The first sequence of alteration, due to hydrothermal topotactic transformation of triphylite into ferrisicklerite and heterosite, is marked by a progressive leaching of Li and by concomitant oxidation of Fe and Mn. The second one involves graftonite, sarcopside and the alteration products of triphylite. It is characterized by a progressive leaching of Ca in oxidizing and hydration conditions. The first step of the second sequence shows an almost complete preservation of the lamellar texture of the primary assemblage graftonite + sarcopside, and produces kryzhanovskite and ferrostrunzite. Kryzhanovskite displays two different chemical compositions, reflecting those of the primary graftonite and sarcopside: the Fe:Mn ratio is preserved. The second step shows the formation of the paragenesis jahnsite-(CaMnFe) + rockbridgeite + lipscombite. In a later episode of alteration, whitmoreite occurs as the result of the auto-oxidation process of ferrostrunzite. This process suggests that a phosphoferrite-like mineral previously occurred in the first step and then transformed into kryzanovskite by auto-oxidation as well.