CHAROITE, CHAROITITES AND ASSOCIATED MINERALS: THE PRODUCTS OF INTENSE METASOMATIC ACTIVITY

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: (K_14.84Na_5.37Ba_1.20Mn_0.21)(Ca_27.37Sr_0.46)Si₇₀(O,OH)₁₈₀(OH_2.79F_1.21) · 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.