Vermiculitization of phlogopite in Metagabbro, central Turkey

Dioctahedral vermiculite occurs in an isolated metagabbro klippe (Kurancali Metagabbro) that belongs to the Central Anatolian Ophiolites from central Turkey. Both the metagabbro and the structurally underlying high-grade meta morphic rocks are intruded by granitic rocks. The Kurancali Metagabbro is c haracterized by its well-developed compositional layering, and the presence of vermiculitized phlogopite rich layers. Petrographic and mineralogic stu dies show that the primary mineral phases in the host rock are diopside, ts chermakitic hornblende, Fe-rich phlogopite, and plagioclase. Secondary mine rals are hornblende, actinolitic hornblende, Fe-rich phlogopite, and vermic ulite. A two-phase history of alteration involving acidic weathering and al kaline metasomatism is suggested for the dioctahedral vermiculite and secon dary Fe-rich phlogopite, respectively. The alteration of phlogopite to dioc tahedral Vermiculite proceeded both along cleavage planes and at crystal ed ges. The vermiculite is colorless to pale yellow with weak pleochroism and shows optical continuity with the parent mineral. Vermiculite flakes, analy zed semi-quantitatively by scanning electron microscope-energy dispersive a nalysis (SEM-EDS) and electron microprobe (EMP), are characterized by parti ally expanded interlayers, K depletion, and Mg and/or Al enrichment. X-ray diffraction (XRD) and differential thermal analysis-thermal gravimetric (DT A-TG) analyses indicate that phlogopite is not a pure phase, although it is the dominant one. The XRD patterns shaw the presence of both dioctahedral vermiculite having dehydrated interlayers and hydroxy-Al interlayers, and i nterstratified phlogopite-vermiculite. The transformation of phlogopite to Vermiculite is thought to represent an initial stage of weathering in an ac idic environment.