Uranopyrochlore — MacDonald Mine, Hybla, Monteagle Township, Hastings County, Ontario, Canada

Pyrochlore group minerals are among the geochemically most interesting of the Precambrian pegmatite accessory phases, accommodating a wide range of large-ion elements — niobium, tantalum, titanium, uranium, thorium, rare earths, calcium, sodium — within a flexible oxide structure that crystallizes in the isometric system. Uranopyrochlore is the uranium-dominant member of the pyrochlore group, with uranium occupying the A-site of the general formula (U,Ca,Ce)₂(Nb,Ta)₂O₆(OH,F). Like euxenite and cyrtolite, it is frequently to entirely metamict in natural specimens: the accumulated radiation from its own uranium content progressively destroys the crystal lattice over geological time, resulting in a glassy, isotropic structure that retains the external crystal form — typically octahedral or modified cubic — while losing internal structural order. The result is a dark brown to black, resinous to subadamantine mass that responds emphatically to a Geiger counter. Field visit records for the MacDonald Mine describe dark brown, glassy massive uranopyrochlore from the mine dumps associated with orange microcline feldspar and dark red-brown cyrtolite zircon crystals — a distinctive association that makes this locality recognizable to experienced collectors. Mindat photo documentation shows the two species together in hand specimens, the dark brown pyrochlore and red cyrtolite set against pale quartz and albite matrix. The MacDonald Mine pegmatite was originally worked for feldspar and later assessed for uranium, with a 40-ton bulk sample reportedly grading nearly 4% U₃O₈ — reflecting the genuine concentration of uranium minerals at this occurrence. This specimen is uranopyrochlore from the MacDonald Mine, a named Hastings County locality with a well-established reference presence in the Canadian mineralogical literature and collector market. Strongly radioactive. Canada domestic shipping only. Approx. specimen size: Approx. specimen weight: 147 grams Approx. specimen activity on an SE International Ranger EXP: 50000 CPM