همایش بین المللی گوهرشناسی و کانیهای صنعتی

صفحه اصلی / دومین همایش بین المللی گوهرشناسی و کانیهای صنعتی

The amethyst-bearing geodes in the andesitic flow at Lisar area (Khalkhal, Iran)

نویسندگان :

Morteza Razmara¹ Arian Almasi²

1- دانشگاه فردوسی مشهد 2- دانشگاه فردوسی مشهد

کلمات کلیدی :

Khalkhal،Amethyst،radioactive minerals

چکیده :

The amethyst-bearing geodes has been found in well-formed crystals in magmatic complexes. These deposits are mainly associated with magmatic-hydrothermal environment. The amethyst geodes observed in the andesitic flows at Lisar area (Khalkhal, Iran) show particular wall-layering infillings with, from the outside inwards, chalcedony, white quartz and large-sized violet amethyst crystals. Amethyst in the study area, is formed as a result of the complex geological processes and geochemical conditions. The geological formation of amethyst involves a complex interplay of different processes (e.g. suitable parent rocks, chemical reactions, hydrothermal activities, suitable spaces and time). The result showed that the first factor that contributes to the formation of amethyst in the studied area, is related to exsolution of magmatic gas and formation of silica-rich solutions. Amethyst crystallized from hot, silica-rich fluids circulating within cavities of parental rocks, geodes, or fractures in existing rock formations. As the hydrothermal fluids cooled, dissolved silica (SiO₂) precipitates and formed quartz. Trace elements of iron (Fe³⁺) and other transition metals present in the fluid substituted silica element (Si) into the growing quartz lattice structure. Natural radiation from radioactive minerals (e.g. alkali feldspars) from surrounding rocks (probably Lisar granite) excited the iron impurities within the quartz lattice structure and enhanced the purple color of amethyst, leading to deeper and more vibrant violet hues. The genesis of amethyst geodes in the study area, is considered as a three-stage process with an early magmatic protogeode formation and a late, low temperature infill of the cavity.