Triangular euhedral crystals with zoning color in nodule tourmaline

Chemical variations and origin of tourmalines in laleh zar granite of Kerman (Southeast Iran)

Sadaf Ahmadi, Zahra Tahmasbi, Ahmad Ahmadi Khalaji, Farhad Zal

Abstract



Tourmalines in the laleh Zar granite of Kerman are found in a variety of forms including: (1) nodule tourmaline ;( 2) radial tourmaline, and (3) vein tourmaline. Microprobe analyses of the tourmalines have revealed a wide compositional variation between schorl and dravite end members. These tourmalines belong to alkali group. Mg/Mg + Fe ratios for radial, nodule, and vein tourmalines range from 0.15 to 0.45, 0.25 to 0.62, and 0.20 to 0.61, respectively; and Ca /Ca + Na ratios for radial, nodule, and vein range from 0.17 to 0.33, 0.15 to 0.35, and 0.07 to 0.3, respectively. All tourmalines have lower Al content than 6 compared with the theoretical value of 6 in ideal schorl and dravite, showing the tourmalines do not contain enough Al to fill the Z-site. The values of X-Vacancy and Al in Y site of the three types of tourmaline are negligible. Nodule tourmalines have chemical and color-zoning patterns, whereas radial and vein tourmalines only have chemical-zoning patterns including significant fluctuations in Al, Fe, Mg, Na, Ca, and Ti. based on petrographic and chemical properties of tourmalines, vein tourmaline has been formed during a transition from magmatic to hydrothermal processes and radial tourmaline has been formed by mixing magmatic-hydrothermal fluids with an external fluid rich in Ca and Mg, whereas nodule tourmaline is probably the result of post-magmatic hydrothermal environment, through the influence of an external fluid that could for example have come from the wall rocks to the nodule.



Keywords


Nodule tourmaline; radial tourmaline; vein tourmaline; schorl; dravite; Laleh Zar granite

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0369-8963 - ISSN ONLINE: 2239-1002