Fragment of dolomite replaced by white spherulite Thalmessite among grey cryptocrystalline Mansfieldite.

Thalmessite, Mansfieldite mineral specimen.

These minerals formed when arsenic-rich solutions reacted with dolomite fragments embedded in brown clay that filled a karst cavity. Talmessite formed from dolomite, and Mansfieldite from clay minerals.

Locality: Sukhonkoye (Au-Hg) deposit, Sarasinsky ore cluster, Altai district, Altai Krai, Western Siberia, Russia