Cyrtolite (Zircon var.) — Massive Crystals | MacDonald Mine, Hybla, Monteagle Township, Hastings County, Ontario

Cyrtolite is the uranium- and thorium-bearing variety of zircon (ZrSiO₄) in which substitution of U⁴⁺ and Th⁴⁺ for Zr⁴⁺ is significant enough to produce a distinctly different character from ordinary zircon — most critically, progressive metamictization as accumulated alpha radiation destroys the crystal lattice over geological time. In fresh specimens, cyrtolite retains the tetragonal prismatic form of zircon with bipyramidal terminations; with increasing metamictization it takes on a swollen, rounded appearance, the curved faces that give it its name (from the Greek for "curved"), and a shift from the sharp, high-relief character of unaltered zircon toward a greasy, resinous lustre and reduced hardness. The colour ranges from reddish-brown to orange-brown and dark brown depending on the degree of radiation damage and iron content. The MacDonald Mine at Hybla in Monteagle Township is one of Hastings County's classic accessory mineral localities, a feldspar quarry that tapped into a rare-element pegmatite carrying uranium, niobium, zirconium, and associated secondary commodities including copper and rare earth elements. Ontario geological survey records note that cyrtolite from the MacDonald Mine occurs in single crystals and masses of crystals up to approximately 2.5 cm in size, frequently coated with hematite. Mindat photo documentation confirms striking red-brown cyrtolite crystals up to 2 cm on matrix, sometimes associated with magnetite, quartz, and the locality's other notable mineral — uranopyrochlore. The two species together make the MacDonald Mine a canonical address for the collector of Canadian niobium-uranium accessory minerals. This specimen presents massive cyrtolite crystal material from the MacDonald Mine dumps. Radioactive owing to the uranium and thorium content incorporated in the zircon structure. Radioactive. Canada domestic shipping only. Approx. specimen size: 70mm x 50mm x 30mm Approx. specimen weight: 149 grams Approx. specimen activity on an SE International Ranger EXP: 9500 CPM