Sodalite syenite from the Bancroft district,

Grenville province, southeast Ontario, Canada

sodalite syenite slice [301 kb]

Fig. 1: This striking sample weighs next to nothing: it is a sawn slice about 1 mm thick, 33x22 mm in area, mounted in a small block of epoxy resin. A polished thin section was cut from the piece, as described below. The bulk of the rock is deep blue sodalite, within which can be seen lesser grains of pale grey to cream nepheline and feldspars, and black flakes of biotite mica. The pale pink to orangey patches appear to represent partial alteration of nepheline and/or sodalite to hydronephelite, an alteration rich in the zeolite mineral natrolite. A representative sample (2163), recovered loose on 14 October 2000.

"Rock of the Month # 281, posted for November 2024" ---

Sodalite syenite

Princess Sodalite Mine, Dungannon Township, Ontario

Sodalite is well-known as a beautiful, deep-blue semi-precious stone. Mineralogically, it is a member of the feldspathoid group, for which the most abundant example is nepheline. Sodalite is a cubic silicate, first described in 1811 from the Ilimaussaq complex in the peralkaline Gardar igneous province of southwest Greenland. The mineral formula is Na8[Al6Si6O24]Cl2. In thin section, it is generally colourless, with moderate relief, and isotropic (black in crossed-polarized light, unlike the related nepheline which is grey). It is found in sodium-rich alkaline igneous rocks, and alters readily to zeolites.

In hand specimen, this feldspathoid takes a range of hues, from the classic deep blue to pink, raspberry, green, white or pale grey. The fluorescent variety hackmanite is quite widely distributed. It occurs, for example, at the De Mix quarry at Mont St. Hilaire, where it exhibits photochromism or tenebrescence (reversible colour change on exposure to light): pink on fresh exposure, fading to colourless within a few minutes; Peterson, 1983; Horvath et al., 2019, pp.414-416). Hackmanite may be pink or purple in a fresh-broken surface. Upon fading, the colour can be restored by exposure to light, most conveniently an ultraviolet light. Other materials that display this phenomenon include spodumene and tugtupite.

This Rock of the Month is from a well-known rockhound favourite, a small quarry and rock shop just east of Bancroft, Ontario (in Dungannon township, Hastings county). The Princess Sodalite Mine is open in the summer months. The property exposes an outcropping of nepheline sodalite syenite. The occurrence was known at least as far back as 1893 (Sabina, 1986). Sawn decorative stone from this quarry was sent to enhance expensive residences in London, England, such as Marlborough House, 1901 (Bedell, 2024) and financier Ernest Cassel's Brook House on Park Lane, 1906 (Sabina, 1986). Marlborough House, built in 1711 for Sarah Churchill, Duchess of Marlborough, is located on The Mall, on the north side of St James's Park in the heart of London.

The nepheline sodalite syenite at the Princess Sodalite mine is part of a suite of igneous rocks for which this part of the Central Metasedimentary Belt of the Grenville province is well-known, giving Bancroft the self-bestowed but not-unreasonable title "Mineral Capital of Canada" (see "Museum Moment" below). According to James (1968), sodalite occurs in veins up to 2 feet wide, controlled by the joint pattern in the nepheline syenite host rock. In part of the quarry, both sodalite and nepheline have been strongly altered to a mixture of zeolites. A two-stage origin of the veins is proposed: 1) nepheline-oligoclase pegmatite forms in tension fractures in host syenite, and 2) albite, nepheline and a NaCl-bearing brine react to form microcline and sodalite. Since at least the 1970s, there have been a few discussions of possible evaporite sediments in the Grenville province in Ontario, New York and New Jersey. These might have provided elements such as Na and Cl (and B, to explain some occurrences of boron minerals). Evaporite sediments are hard to find in the geological record, beyond a Grenville age of circa 1200 Ma.

The Princess Sodalite locality has been noted in many field guides and other publications over the years, e.g., MacIntyre et al., 1967; Stonehouse, 1967; Baer et al., 1972; Hogarth et al., 1972; Gittins and Lumbers, 1972; Currie, 1976; Storey and Vos, 1981; Sabina, 1982; Masson, 1982. The area has been noted for exotic minerals, and nice specimens of minerals common to rare, for many years. Thus Bancroft features in a range of rock-collecting guides (e.g., Hewitt, 1969; Sabina, 1986, pp.11-14; Fouts, 1998).

Mineralogy of this rock: A preliminary examination of the thin section provides the following estimate of modal mineralogy (the area or volume proportions of each mineral in a rock):

  • Sodalite, 70%
  • Microcline K-feldspar, 13%
  • Biotite mica, 5%
  • Nepheline, 5%
  • Plagioclase feldspar (albite), 4%
  • Natrolite variety hydronephelite, 2%
  • Calcite, 1%
  • Magnetite, Trace
  • Pyrite, Trace
  • Zircon, Trace
  • Pyrrhotite (?), Trace
This result closely matches inspection of an earlier section made from this rock in 2001. The sodalite is rather featureless in section! It is colourless and shows no zonation nor twinning, though it does contain tiny, colourless elongate inclusions, no more than 60-70 microns long, that display first and second-order interference colours, with straight extinction and length-slow orientation (the birefringence could match cancrinite, but not the orientation).

Figures 2-6 will illustrate the principal minerals, at 50X magnification, long-axis field of view 1.7 mm, all in crossed-polarized transmitted light, except for (4) that shows biotite mica in plane-polarized light. The polars are not perfectly crossed here, so the sodalite appears dark grey and rather featureless (rather than black), in comparison with nepheline and feldspars.

sodalite syenite PTS sodalite syenite PTS

Figs. 2-3: The view through the thin section is quite unlike the hand specimen, in that the most abundant mineral, lovely blue sodalite, appears colourless, and furthermore is black in the crossed-polarized light that reveals more detail in feldspars and some of the other minerals! Left: A twinned K-feldspar crystal enclosed by sodalite. Right: an elongate nepheline crystal in sodalite host. For the microscopist, the nepheline is distinct from its host in a) having higher refractive index, b) is not isotropic, though the birefringence is low first-order, no higher than quartz, and c) has length-fast orientation.

sodalite syenite PTS sodalite syenite PTS sodalite syenite PTS

Figs. 4-6: Left: Rich brown flake of biotite mica. This ragged flake has been deformed, as seen by the changing orientation (and so difference in intensity of brown hue through pleochroism). It is hosted by clear sodalite, which also contains small grains of feldspar and (above and left of the mica) a grain of nepheline which has higher relief than its host. Centre: Detail of one of the pink patches in the hand specimen. An angular relict grain of nepheline is clearly visible on the left. Near the right edge of the view are small granules of calcite, which appear dark in this orientation. The bulk is a mass of small rather irregular grains, thought to be a variant of the zeolite natrolite *, formed by alteration of nepheline and sodalite. Right: A coarse nepheline crystal encloses a small untwinned feldspar grain, in the host sodalite, which appears to the left.

* Hydronephelite: this secondary alteration product seems to lack the classic radial- acicular crystal habit of natrolite, when it is found with other zeolites in gas cavities in lavas. Now, "hydronephelite" (or hydronepheline) is not an approved mineral species. First described at Litchfield, Maine, it is an alteration product of feldspathoids, and consists largely of natrolite. It has variously been described in: the Ice River complex of British Columbia (Allan, 1914); in Paraguay (Eckel et al., 1959); and elsewhere. Back in the Bancroft area, hydronephelite is also reported at the nearby Davis nepheline pegmatite (Sylvester and Anderson, 1976) and further east near the old corundum workings at Craigmont (Moyd, 1950). Near the Stony Lakes, south of Bancroft, at the Blue Mountain nepheline mine, pods of secondary minerals such as cancrinite and hydronephelite are quite common, and visible in freshly-blasted faces for their pink colour within the white host rock (Edgar, 1968). Sodalite may be replaced by mixtures of natrolite and nordstrandite, Al(OH)3. as noted in the Lovozero alkaline complex in Arctic Russia.

Sodalite further afield:

Sodalite and other feldspathoids are generally restricted to silica-poor igneous rocks, nepheline syenites and the like (Taylor, 1967). One very unusual rock from Greenland, a naujaite, is a flotation cumulate, where sodalite crystallized and, being less dense than the parent magma, floated upwards and concentrated towards the top of the magma chamber! This is the reverse of typical cumulates where denser phases such as chromite, magnetite or olivine settle towards the base of the intrusion. The curious magma chamber processes were described in detail by Ferguson (1970), and by Larsen and Sorensen (1987). Naujaite appears mostly in rather obscure volumes of petrology (e.g., Kemp, 1929; Johannsen, 1938). but these alkaline rock suites contain a wide range of uncommon to rare elements such as uranium, rare earths and zirconium. In petrologic terms, the Gardar province of southwest Greenland is one of the great mineral fields of the world, with some outlandish petrology, where minerals rare elsewhere may assume rock-forming proportions (e.g., Bøggild, 1953). Sodalite is found in many alkaline igneous complexes, including the Poudrette and De Mix quarries of Mont St-Hilaire, one of the Monteregian Hills east of Montreal (Horvath et al., 2019). Sodalite and related minerals are found in numerous occurrences of alkaline plutonic and volcanic rocks. The latter include the Monte Somma-Vesuvius volcanic field in Italy (Balassone et al., 2016). Moving "off-world", sodalite is also found as a secondary phase in chondrite meteorites, occurring with other alteration products such as nepheline, hedenbergite and fayalitic olivine (Wang and Hsu, 2008).


MUSEUM MOMENT #10

The town of Bancroft hosts the Rockhound Gemboree , the largest rock, mineral and fossil show in Canada, at the end of July to the start of August each year, and 2024 saw the 59th meeting, which was a great success. The town also boasts a lovely little museum with some exquisite displays, the Bancroft Mineral Museum, 8 Hastings Heritage Way, Box 1749, Bancroft, Ontario K0L 1C0. Well worth a visit! Admission is $5 (children under 15 free) and the building, which is the old railway station, is shared with the town offices. The 1,200 square-foot show room features over 400 local specimens (and there is a spare case for travelling exhibits). The displays include rocks, minerals, local ores and more. The fluorescent mineral display is excellent, with local material, and high-quality specimens from far away, all seen in long-wave, medium-wave and short-wave ultraviolet light. The community also has the long-running Bancroft Gem & Mineral Club, a knowledgeable group who host talks and conduct field trips (in the appropriate seasons), and maintain the displays.


Acknowledgements: Thanks to Chris Fouts for valuable editorial comments!

REFERENCES

Allan,JA (1914) Geology of Field Map-area, B.C. and Alberta. GSC Memoir 55, 312pp.

Baer,AJ, Frarey,MJ and Ayres,LD (1972) The Geology of the Canadian Shield between Winnipeg and Montreal. IGC 24, Montreal, Excursion Guidebook A35-C35, 105pp.

Balassone,G et al. (2016) Sodalite-group minerals from the Somma-Vesuvius volcano (Naples, Italy): a combined EPMA, SIMS, and FTIR crystal-chemical study. Can.Mineral. 54, 583-604.

Bedell,RL (2024) The vastly contrasting mineralogy and formation of pegmatites in Bancroft, Ontario and Maine, USA. Presentation 77 to Kawartha Geoscience Network (Kawartha and Region Earth Sciences, Engineering and Metallurgy Network, KREEM), Peterborough ON, delivered via Zoom to a mixed live and on-line audience (06 February).

Bøggild,OB (1953) The Mineralogy of Greenland. Meddelelser om Grønland 149.3, 442pp.

Currie,KL (1976) The Alkaline Rocks of Canada. GSC Bull. 239, 228pp. plus map.

Eckel,EB, Milton,C and Sulsona,PT (1959) Geology and mineral resources of Paraguay - a reconnaissance. USGS Prof.Pap. 327, 110pp. plus 2 maps.

Edgar,AD (1968) Mineralogy of a zoned replacement body from the Blue Mountain litchfieldite, Peterborough county, Ontario. Amer.Mineral. 53, 1048-1053.

Ferguson,J (1970) The significance of the kakortokite in the evolution of the Ilimaussaq intrusion, South Greenland. Grønlands Geologiske Undersøgelse Bull. 89, 193pp.

Fouts,C (1998) Bancroft & District Mineral Collecting Guidebook. Bancroft & District Chamber of Commerce, 50pp.

Gittins,J and Lumbers,SB (1972) Alkalic rock complexes and carbonatites of Ontario and part of Quebec. IGC 24, Montreal, Excursion Guidebook A53-C53, 41pp.

Hewitt,DF (1969) Geology and Scenery, Peterborough, Bancroft and Madoc Area. ODM Geological Guide Book 3, 114pp.

Hogarth,DD, Moyd,L, Rose,ER and Steacy,HR (1972) Classic Mineral Collecting Localities in Ontario and Quebec. IGC 24, Montreal, Excursion Guidebook A47-C47, reprinted as GSC Misc.Pap. 37, 79pp.

Horvath,L, Gault,RA, Pfenninger-Horvath,E and Poirier,G (2019) Mont Saint-Hilaire: History, Geology, Mineralogy. Canadian Mineralogist Spec.Publ. 14, ix+634pp.

James,RS (1968) An occurrence of sodalite at the Princess quarry, Bancroft, Ontario. Can.Mineral. 9, 574.

Johannsen,A (1938) A Descriptive Petrography of the Igneous Rocks, Volume IV. Part I, the Feldspathoid Rocks, and Part II, The Peridotites and Perknites. University of Chicago Press, 523pp.

Kemp,JF (1929) A Handbook of Rocks, for use without the microscope. D. Van Nostrand Company, Inc., New York, 6th edition, 2nd printing, 300pp.

Larsen,LM and Sorensen,H (1987) The Ilimaussaq intrusion - progressive crystallization and formation of layering in an agpaitic magma. In `Alkaline Igneous Rocks' (Fitton,JG and Upton,BGJ editors), Geol.Soc.Spec.Publ. 30, Blackwell Scientific Publications, 568pp., 473-488.

MacIntyre,RM, York,D and Moorhouse,WW (1967) Potassium-argon age determinations in the Madoc-Bancroft area in the Grenville province of the Canadian shield. Can J. Earth Sci. 4, 815-828.

Masson,SL (1982) Geology and mineral deposits of the Bancroft area, Eastern Part, southern Ontario. OGS Map P2524, 1:10,000 scale.

Moyd,L (1950) Structure of the corundum deposit at Craigmont, Renfrew county, Ontario. Proc.GAC 2, 51-56.

Peterson,RC (1983) The structure of hackmanite, a variety of sodalite, from Mont St-Hilaire, Quebec. Can.Mineral. 21, 549-552.

Sabina,AP (1982) Some rare minerals of the Bancroft area. Mineral.Record 13 no.4, 223-228.

Sabina,AP (1986) Rocks and Minerals for the Collector: Bancroft - Parry Sound area and Southern Ontario. GSC Misc.Rep. 39, 182pp.

Stonehouse,HB (1967) Guidebook for field trip to the Grenville of southeastern Ontario. Institute on Lake Superior Geology, East Lansing, MI, 74pp.

Storey,CC and Vos,MA (1981) Industrial Minerals of the Pembroke-Renfrew area, Part 2. OGS MDC 22, 214pp. plus OGS map P2209, 1:126,720 scale.

Sylvester,GC and Anderson,GM (1976) The Davis nepheline pegmatite and associated nepheline gneisses near Bancroft, Ontario. Can.J. Earth Sci. 13, 249-265.

Taylor,D (1967) The sodalite group of minerals. Contrib.Mineral.Petrol. 16, 172-188.

Wang,Y and Hsu,W (2008) Petrographic study of Ningqiang sodalite. Meteoritics & Planetary Science 43, A165.

Graham Wilson, 31 October-02 November, 07-08 November 2024

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