Captions for Mont Saint-Hilaire photographs on display

[Jeanne, I would group the two photos below with captions as follows. Depending on the placement of the photos the "above" and "left" may not be needed]:

Above, crystal-lined cavities exposed in 1998 in the Poudrette pegmatite. This is the largest pegmatite encountered in the Poudrette quarry. Photo: L. Horváth

Left, collector Gilles Haineault with a large specimen of pseudomorphs of analcime after natrolite recovered from the Poudrette pegmatite. Photo: L. Horváth.

[Jeanne, This next photo is separate.]

Field crew from the

Canadian Museum of

Nature collecting

sérandite from a

pegmatite pocket in 1973.

Photo: G. Runnells

A pegmatite cavity lined with crystals of
microcline, sérandite and polylithionite. This
cavity, exposed in 2003, is 1.3 metres across,
and over 2 metres deep. Photo: P. Tarassoff

SYENITIC PEGMATITES

Pegmatite cavities are the chief source of the fine, crystallized mineral specimens for which Mont Saint-Hilaire is famous. The pegmatites range from dykes up to tens of meters in length and 3 meters in thickness, to irregular, pipe-like and spheroidal bodies up to a few meters in size. Compositionally, they range from pegmatites with a mineralogy similar to the host syenites, to pegmatites rich in carbonates and devoid of pyroxenes and amphiboles. Most have been subject to varying degrees of hydrothermal replacement and supergene alteration.

The primary minerals in nepheline syenite pegmatites are typically microcline, albite, irine, nepheline, analcime, eudialyte, and kupletskite, with local concentrations of catapleiite, s andite and rhodochrosite. In later stages of deposition, hydrothermal activity has introduced natrolite, polylithionite, sodic amphibole, second-generation albite and irine, titanium, zirconium and niobium silicates, and rare-earth carbonates. Pseudomorphs formed by the replacement of earlier deposited minerals are common.

Caption: A sodalite xenolith, about 0.5 metres across,
in nepheline syenite, Poudrette quarry. Photo: P.
Tarassoff

SODALITE XENOLITHS

Consisting mainly of coarse-grained sodalite, these xenoliths
represent the most alkaline environment in the East Hill suite.
They are the source of a large number of rare minerals, including
five new species and several that are water-soluble. The xenoliths
are very localized in occurrence. Their origin is uncertain.

Caption: A marble xenolith, about 3 metres in length, in nepheline syenite, Poudrette quarry. Photo: L. Horváth

.

MARBLE XENOLITHS

These range from true marbles composed almost entirely of calcite, to siliceous marbles rich in sodic and calcic silicates. Pectolite is the most common silicate, forming fibrous intergrowths with calcite, and frequently lining cavities as well-formed crystals. Other common minerals in the cavities are vesuvianite, fluorapophyllite, sodic amphiboles such as arfvedsonite, and micas such as phlogopite and tainiolite. A number of very rare minerals such as raite also occur in the cavities.

Caption: This hornfels is part of a huge block of shale that was engulfed by magma. Note the now near vertical bedding planes preserved in the hornfels. Field of view about 25 metres. Poudrette quarry. Photo: P. Tarassoff

HORNFELS

During the emplacement of the East Hill suite, huge blocks of Lorraine shale slumped into the magma and were metamorphosed into hornfels. The hornfels are crisscrossed by mineralized fissures where hydrothermal fluids penetrated fractures and bedding planes. The most common minerals in open fissures are feldspars, calcite, dolomite, rutile, quartz and pyrite. Rarer minerals in the hornfels include the tourmaline group mineral, dravite.

Caption: Igneous breccia in the Poudrette quarry. Photo: P. Tarassoff

IGNEOUS BRECCIAS

Igneous breccias consist of irregular to rounded fragments of hornfels and older igneous rocks from the Mont Saint-Hilaire pluton cemented by fine-grained nepheline syenite. In the Poudrette quarry they occur in localized zones. The breccias occasionally contain irregular cavities which are a source of well crystallized, often rare minerals.