2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 168-8
Presentation Time: 3:30 PM


DUTROW, Barbara L., Dept. of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101 and HENRY, Darrell J., Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, dutrow@lsu.edu

Charoite, a rare and unusual alkali silicate, is characterized by intense purple hues and a fibrous habit. Similar to jadeite (and jadeitites), it is a popular gemstone. Strongly folded fibers form continuous mats that comprise up to 90% of the rock (charoitite) or enclose coexisting phases such as Fe3+ -rich K-feldspar, dalyite, aegirine, potassic magnesio-arfvedsonite, tinaksite, canasite, yuksporite, Sr-Ba carbonates, sulfides, thorite, pectolite, and/or quartz, commonly as veins. Complexly zoned feldspars, up to 1 cm, commonly include charoite needles suggesting contemporaneous formation. Charoite has a single recorded occurrence, adjacent to the Murun alkalic igneous complex, Chara River Valley, Russia, where it is restricted to pods within fenitic host rocks. Because of the spatial location and low temperature stability (200-250°C), the formation of charoite is attributed to metasomatism by fluids related to igneous activity.

Detailed EMP analyses of charoite elucidate the chemical composition and substitutional mechanisms. Charoite is rich in alkalis, silica, and H2O but lacks Al. A small but significant amount of Mn2+ is present in all samples and is probably responsible for the purple coloration. Chemical data, normalized on the basis of 70 Si, exhibit a limited compositional range across 8 samples (in apfu): K 13.80-15.60; Na 4.26-6.16; Ca 26.19-28.17; Ba 0.90-1.39; Sr 0.17-0.59; Mn 0.07-0.30; F 0.80-1.54. Al, Ti, Fe, Mg, Zr and Th are ≤ 0.02 wt %. These compositional data yield an average formula: (K14.84Na5.37Ba1.20Mn0.21)(Ca27.37Sr0.46)Si70(O,OH)180(OH2.79F1.21) · nH2O. Chemical variations are primarily described by the homovalent substitutions: NaK-1, BaCa-1, SrCa-1, Mn2+Ca-1, and FOH-1.

Charoite-bearing samples contain strongly cathodoluminescent (CL) minerals. Charoite displays an orange-yellow CL, dalyite luminesces azure blue, and Kfs luminesces crimson red. The orange-yellow CL denotes the presence of Mn3+ and the absence of Fe2+. Ti4+ is responsible for the blue CL of dalyite. Significant amounts of tetrahedral Fe3+ replacing Al3+ result in the spectacular red CL of Kfs. These chemical studies provide insights into the composition of charoite and establish the necessary prerequisite for quantification of mass transport for the unique conditions of charoite formation.