Charoitite (charoite-dominated potassic metasomatite) from the Cretaceous of Siberia. (public display, Yakutsk Public Unified Museum of History & Northern Folk Culture, Yakutsk, Siberia, Russia)

The rare, impressively attractive, purple-colored mineral charoite is only known from a relatively small area in Siberia. Several odd, large igneous intrusions have been emplaced in Siberia’s Baikal-Aldan Belt. One of these, the Murun Complex, consists of several plutons. Emplacement of the Little Murun Pluton (Malyy Murun Pluton) has resulted in significant contact metamorphism of surrounding rocks. In the southern and southeastern contact aureole of the Little Murun Pluton is a charoitite-carbonatite complex.

The charoitites are charoite-dominated contact metamorphic rocks. Metasomatism refers to rocks that have been altered by significant input of elements from an outside source. Because that’s primarily what contact metamorphism is about, such rocks can be called metasomatites. Some of the metasomatites in this area are purple-colored (“Sirenevyy Kamen” - the “Purple Stone” deposit). The purple material is the very rare mineral charoite. Charoite is a “garbage can” mineral, and the chemical formula assigned to it by geologists varies from publication to publication. Here’s one of the formulas I’ve seen, randomly picked: (K,Na)3(Ca,Sr,Ba,Mn)5Si12O30(OH·F)·3H2O - hydrous potassium sodium calcium strontium barium manganese hydroxy-fluorosilicate.

Age: Aptian Stage, Early Cretaceous, 115-120 Ma