Mount Saint-Hilaire — Quebec's Rare Mineral Mecca
Mount Saint-Hilaire — Quebec's Rare Mineral Mecca
An alkaline intrusive complex producing more type-locality minerals than almost any other site
Mont Saint-Hilaire — also rendered Mount Saint-Hilaire in English-language references — is an alkaline intrusive complex of the Monteregian Hills of southern Quebec, Canada, that has produced one of the most spectacular suites of rare and unusual minerals known from any single locality on Earth. Over 365 mineral species have been identified from the Mont Saint-Hilaire pegmatites, sodalite syenite, and associated rocks, with more than 50 of these species first described from Mont Saint-Hilaire material — making the site one of the world's premier type localities. The site is a recognised pilgrimage destination for serious mineral collectors and a source of significant specimens for major museum collections worldwide.
Geology
Mont Saint-Hilaire is one of the eight or nine Monteregian Hills, a chain of alkaline intrusive bodies of Cretaceous age (approximately 125 million years old) that intruded into the Palaeozoic sedimentary cover of the St Lawrence Lowlands of southern Quebec. The Mont Saint-Hilaire pluton is composed principally of nepheline syenite — an alkaline igneous rock relatively poor in silica and rich in alkalis (sodium and potassium) — that crystallised at depth and has subsequently been exhumed by erosion. Within the syenite, late-stage residual fluids and crystallising magmas produced pegmatites and miarolitic cavities that hosted the exceptional mineral assemblages for which the site is famous.
The unusual chemistry of the Mont Saint-Hilaire system — with high concentrations of alkalis, beryllium, niobium, zirconium, rare earth elements, and other incompatible elements — produced minerals incorporating these elements in unusual proportions and combinations. The result is a mineralogical inventory dominated by rare and exotic species rarely encountered elsewhere.
Notable species
Mont Saint-Hilaire is the type locality for a remarkable list of minerals. Notable type-locality and notable-occurrence species include serandite (a pink alkali pyrosilicate), eudialyte (a complex zirconium silicate, also known from Greenland and the Kola Peninsula), astrophyllite (a brown to bronze titanium silicate forming spectacular bladed crystals), catapleiite (a sodium zirconium silicate), various sodalite group minerals (particularly hackmanite, which displays tenebrescence — reversible photochromism), genthelvite, leifite, weloganite (a strontium zirconium carbonate), and dozens of other rare species. Gem-quality material is occasionally encountered in nepheline, sodalite (including transparent gem hackmanite), analcime, and a small number of other species.
Collecting history
Mont Saint-Hilaire was an active commercial quarry for road aggregate and decorative stone for much of the twentieth century, with the quarry operations periodically intersecting mineralised pegmatite zones and producing collector specimens that reached the international market through dealer channels. The most prolific period for new species discoveries was the 1960s through the 1990s, with quarry operations supplying the access that mineralogists and collectors required. The principal commercial quarry has since been closed, and current collecting access is significantly restricted; specimens currently in circulation derive principally from earlier production and from limited continuing scientific access.
In the trade
Mont Saint-Hilaire specimens are traded principally in the high-end mineral specimen market through specialist dealers and at major mineral shows (Tucson, Denver, Munich, Sainte-Marie). Provenance is decisive for value, and specimens with documented field-collecting provenance from named pockets and dates command premiums over comparable material without provenance. Mineralogical reference works including the Mineralogical Record and Canadian Mineralogist have published extensively on the locality, providing the documentary base for collector and scholarly study.