The Hirsizderemagnesitedepositis foundin the upperMiocene-Pliocene lacustrinesedimentsofthe Cameli formation in the western Aegeandepression zone, whichwas upliftedto form a broad anticlineand in which consecutivefaults and fractures developeddue to the neotectonic activities. Thick alternation of carbonatecementedand weaklycementedconglomerate, sandstone,mudstoneintercalated with a fewthindolomiteand magnesitelayersis exposedat the lowerpartof the sequence. Theseunitsareoverlainby magnesiteanddolomite in associationwiththin claystoneand sandstone layersandlenses. Greenandclaretred-brownsmectiteis observedin detritalmaterialsas well as in sandycarbonateunits,and isaccompaniedbyserpentine,amphibole, illite,feldsparand quartz,whereassepioliteis developedinthe form of3-5 em thick,yellowandgreenishbrowncoloredveins,or asthin filmsliningparallelto theveinaxiswithinthe magnesiteunit. SEMstudies showthat sepiolitefibersare grown in theformof meshworks and bridgesbetweenrelictcarbonatemineralgrains. Thepresenceofplantstem imprints, and the abundance of gastropod and ostracode shells in carbonate units indicate shallow and alkaline lake conditions. Field observations and detailed mineralogical, chemicaland micromorphological determinations indicate that sepiolite is formed by diagenetical transformation of magnesiteduring the movementof silica-richmeteoricwater through the fractures of carbonate units under near-surface alkalineconditions.