Coffinite & Volborthite - Shinarump Conglomerate, Navajo County, Arizona, USA

U(SiO₄) · nH₂O        Cu₃(V₂O₇)(OH)₂ · 2H₂O

Coffinite and Volborthite are two intriguing minerals that are occasionally found within the Shinarump Conglomerate, a geologically significant member of the Chinle Formation, widespread in the Colorado Plateau region of the southwestern United States. The Shinarump Conglomerate is known for its uranium-vanadium deposits, providing a perfect environment for the formation of these two minerals, which have distinct compositions and origins but often share geological contexts.

Coffinite ($U(SiO_4)_1-x(OH)_4x$) is a uranium silicate mineral and an important secondary uranium ore. It forms under reducing conditions within uranium-rich deposits, replacing primary uranium minerals like uraninite. In the Shinarump Conglomerate, coffinite typically occurs as dark, opaque masses or fine disseminations within the sandstone matrix. The mineral’s formation is facilitated by the interaction of silica-rich groundwater with uranium-bearing deposits under specific pH and redox conditions. The conglomerate’s permeability and its organic matter content create an ideal reducing environment, stabilizing uranium in its tetravalent form and enabling the formation of coffinite.

Coffinite’s significance lies not only in its role as a uranium ore but also in its contribution to understanding the mobility of uranium in sedimentary systems. Specimens from the Shinarump Conglomerate are of interest to researchers studying diagenetic processes and the behavior of uranium in subsurface environments.

Volborthite ($Cu_3(VO_4)_2 \cdot 3H_2O$) is a copper vanadate mineral known for its vibrant green to yellow-green color and characteristic platy or tabular crystal habit. In the Shinarump Conglomerate, volborthite forms as a secondary mineral in oxidized zones of vanadium-rich deposits, often associated with other vanadium minerals such as carnotite. Its formation is driven by the interaction of vanadium-bearing fluids with copper-rich zones, a process often enhanced by the oxidizing conditions present in the conglomerate’s upper layers.

Specimens of volborthite from this locality are prized for their bright coloration and fine crystal aggregates, which often appear as crusts on sandstone surfaces. These specimens not only appeal to collectors for their aesthetic value but also provide insights into the geochemistry of vanadium in sedimentary uranium deposits.

The Shinarump Conglomerate, composed of coarse sandstone, conglomerate, and mudstone, serves as an excellent host rock for uranium-vanadium mineralization. Its high permeability and organic content create chemical environments conducive to both reducing and oxidizing reactions, leading to the formation of coffinite in reducing zones and volborthite in oxidized areas. The spatial association of these minerals highlights the dynamic geochemical gradients within the conglomerate.

Approx. specimen size: 50mm x 40mm x 32mm

Approx. specimen activity on an SE International Ranger EXP: 3000 cpm