Allanite-(Ce) - MacDonald Mine, Monteagle Township, Hastings Highlands Municipality, Hastings County, Ontario, Canada

(CaCe)(AlAlFe²⁺)O[Si₂O₇][SiO₄](OH)

Allanite‑(Ce) from the historic MacDonald Mine in Monteagle Township, Hastings Highlands, Ontario, is a rare-earth–rich sorosilicate in the epidote group, notable for its high cerium (Ce) content. Geologically, the MacDonald Mine sits within the Mesoproterozoic Central Gneiss Belt of the Grenville Province. It was primarily exploited as a feldspar and uranium prospect, with pegmatites and hydrothermal veins intruding metamorphic host rocks such as gneiss and quartzite. Allanite‑(Ce) occurs as accessory crystals associated with these felsic pegmatitic bodies, forming during late-stage magmatic differentiation and rare-earth–bearing fluid exsolutions. The mineral’s presence reflects the complex interplay of REE-enriched fluids and the granitic pegmatite environment common to this part of Ontario.

Crystallographically, allanite‑(Ce) exhibits monoclinic prismatic habits, of a brown to black colour with resinous to vitreous luster, often appearing as prismatic grains or granular masses in the rock matrix. Chemically, it has the formula CaCeFe²⁺Al₂(Si₂O₇)(SiO₄)O(OH), with Ce dominating the rare-earth element group. It may incorporate thorium and uranium, which can render it slightly radioactive and occasionally metamict, disrupting its structure over geological time. In the MacDonald Mine, allanite‑(Ce) is commonly found alongside other rare‑earth and uranium minerals such as uranpyrochlore (ellsworthite), feldspar, niobium‑bearing phases, and quartz, reflecting its formation in an enriched, late‑stage mineralizing system  This assemblage makes the MacDonald deposit a valuable locale for studying rare‑earth geochemistry in Grenville‑aged pegmatitic environments.

Approx. specimen size: 60mm x 40mm x 25mm

Approx. specimen weight: 70.8 grams

Approx. specimen activity on an SE International Ranger EXP: 1200 cpm