The REE-Induced Absorption and Luminescence in Yellow Gem-Quality Durango-Type Hydroxylapatite from Muránska Dlhá Lúka, Slovakia

Abstract

In talc-magnesite veins in serpentinite near Muránska Dlhá Lúka (MDL), Slovakia, yellow euhedral to subhedral crystals apatite of a gem quality occur. It has a composition of hydroxylapatite with F− varying between 0.29 and 0.34 apfu, Cl− in range of 0.02–0.05 apfu and calculated OH− content between 0.62–0.68 apfu. Moreover, [CO3]2− molecules were identified by FTIR and Raman spectroscopy. MDL apatite contains only up to 0.003 apfu As5+ and Si4+ substituting for P5+; Ca is substituted by small amount of Na, Fe2+, Mn2+ (all up to 0.006 apfu), and Rare Earth Elements (REE—in total up to 0.017 apfu). Compared to trace-element composition of similar apatites from Durango, Mexico, the REE content in MDL apatite is around ten times lower with Nd > Ce >> La, its chondrite-normalized REE pattern has almost a horizontal slope and larger negative Eu anomaly. The MDL apatite is richer in Mn, Pb and Li, but poorer in As, V, Th and U. The concentrations of Sr and Y are similar. In the optical absorption spectra, the most prominent bands are at 585–590 nm (Nd3+) and between 600 and 800 nm (Mn2+, Ce3+-SiO3− photochromic center and Nd3+). The photoluminescence spectrum of MDL apatite shows bands between 550 and 620 nm (Dy3+, Sm3+, Pr3+ and also Mn2+) which likely enhance its yellow color. MDL hydroxylapatite likely formed from fluids derived from granitic rocks as evidenced by the chondrite-normalized REE patterns, Li, Mn and Y concentrations. The Sr content reflects the host-rock serpentinite composition. Fluids for its crystallization were likely derived from Muráň complex orthogneisses by the Alpine deformation and recrystallization in greenschist to lower amphibolite facies.