Canadian Rockhound: Borates and other Minerals from the Evaporates of Nova Scotia

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Borates and other Minerals from the Evaporates of
Nova Scotia

By Ronnie Van Dommelen

In the Carboniferous, about 350-335 million years ago, Nova Scotia lay near the equator and much of the province was under water, covered by the Windsor Sea. The climate was very hot and dry, leading to evaporation of the inland sea. Today we see evidence of this sea in the extensive deposits of limestone, gypsum, anhydrite, and salt found in the province. The limestone contains various fossilized shells, corals, and crinoids. The purpose of this article, however, is to focus on the gypsum/anhydrite and salt deposits and the minerals that are associated with them.

As shown in Figure 1, large areas of Nova Scotia are underlain by the evaporate rocks (limestone, gypsum, and salt). Several mines take advantage of these deposits near Windsor, Milford, Pugwash, and Nappan. In fact the Milford mine (Figure 2) is the largest gypsum mine in the world, producing about 3 million tonnes annually. When combined with the outputs of several other mines in the province, Nova Scotia supplies 25% of the North American market for gypsum which is used primarily in the housing industry for wallboard.

Map of evaporite deposits
Figure 1: Map of Nova Scotia showing deposits of limestone, gypsum, and salt.
East Milford gypsum quarry
Figure 2: The gypsum quarry at East Milford, Nova Scotia.

With all this gypsum one might think there are fantastic crystals of selenite everywhere, but alas, this is not the case. Nonetheless, the gypsum can be quite attractive. The fibrous variety of gypsum, informally called satin spar, is shown below (Figure 3). There are also several interesting minerals that are found within the gypsum that are of interest to the collector.

Orange selenite
Figure 3: Orange satin spar from Cheverie, Nova Scotia.

History

A considerable amount of scientific work was performed in the mid 1800's by Henry How, professor of chemistry and natural history at King's College, which at the time was located in Windsor, Nova Scotia. He published several papers on the minerals he encountered in the gypsum (as an aside, How also discovered the zeolite mordenite from the basalts of the Bay of Fundy).

The first interesting mineral found by How in the gypsum/anhydrite was ulexite, in 1857, which he referred to as natroborocalcite. At the time it was considered an extremely rare mineral, having been previously found only at Tarapaca, Peru. Over the years, he reported the mineral from several localities in the Windsor area. It has since been reported in Cape Breton as well.

In 1861, he reported on another new mineral which he called cryptomorphite. This material has caused much confusion in the past, however it is now thought that he was studying impure ginorite.

As a grand finale to his study of the evaporates, his last discovery was the most important. In 1868 he reported on a new borate mineral which he called silicoborocalcite. This would later be named howlite (Ca₂B₅SiO₉(OH)₅), by Dana, in his honor (Figure 4). He initially reported finding small massive white nodules, however in later reports larger specimens had been found, "up to the size of a man's head". More recently rare crystals of howlite have been found in Cape Breton.

Howlite
Figure 4: Howlite crystals naturally etched from matrix.

Mineralogy

About 15 km north of Windsor is the small town of Cheverie. Here, along the shore of the Avon River the gypsum/anhydrite outcrops in a significant way, and is called White Head for obvious reasons. In the gypsum there are small amounts of pyrite, marcasite, fluorite, calcite, and danburite. Celestite is gently etched from seams of bright orange satin spar. In addition to these minerals a small amount of another borate, probertite (NaCaB₅O₇(OH)₄·3H₂O), has been found (Figure 5). It forms extremely delicate, bright white, acicular balls in the gypsum. They are so delicate in fact, that the same water that etches them from the gypsum also mats the delicate needles.

Figure 5: Bright white-matted ball of probertite.

In addition to probertite, balls of anhydrite needles can also be found in the gypsum at Cheverie (Figure 6). These are not nearly as bright white as the probertite, being approximately the same color as the matrix. They are larger too, commonly growing up to a centimeter in diameter. When broken, they exhibit a silky luster. The centers of the balls are often hollow.

Figure 6: Radiating ball of fibrous anhydrite.

One other mineral that deserves mention is inyoite. Goodman, states that "beautiful colourless crystals" were found in the Wentworth area of Hants County. His only reference was the personal communication with an M.F. Goudge in 1955. Unfortunately, the author and other current collectors have not seen any inyoite specimens from Nova Scotia. Further investigation is required to find these potentially attractive specimens.

If we move now to the northwestern part of the province, we find halite rather than gypsum/anhydrite. Halite is much more soluble in water than gypsum, so as the Windsor Sea evaporated the gypsum precipitated out first. Then as more water evaporated the salt was forced to precipitate out forming beds on top of the gypsum. There are two salt mines currently in operation: the Pugwash mine uses the room and pillar method of extraction, and the Nappan mine uses a brine method to bring the commodity to the surface.

While halite is typically colorless, occasionally blue specimens are encountered at the Pugwash mine. The blue color is due to natural radiation that causes submicroscopic particles of sodium metal to form within the halite. From the Nappan mine at least one specimen of chambersite (Mn₃B₇O₁₃Cl) has been found. The pictured specimen shown in Figure 7 is from the Fundy Geological Museum in Parrsboro, Nova Scotia, where it is on display.

Boracite
Figure 7: A sharp 1 cm crystal of chambersite from the Nappan salt mine. Photo courtesy of the Fundy Geological Museum.

In conclusion, it is clear that the gypsum and anhydrite deposits are important to the geology of Nova Scotia. These deposits have produced small amounts of interesting minerals, including several borates. All the result of an ancient sea.

Acknowledgements

My thanks Ken Adams and Kathy Goodwin from the Fundy Geological Museum for the boracite photo, Ralph Stea for use of the Milford Quarry photo, as well as Terry Collett and Doug Wilson for much other information relating to this article.

References

N.R. Goodman, "Gypsum in Nova Scotia and its Associated Minerals," Geology of Canadian Industrial Mineral Deposits, 1957.
H. How, "On the Occurrence of Natro-boro-calcite with Glauber-Salt in the Gypsum of Nova Scotia", Silliman's Journal, pp. 230-235, 1857.
H. How, "Natro-boro-calcite and another Borate occurring in the Gypsum of Nova Scotia", Silliman's Journal, pp. 9-13, 1861.
H. How, "Contributions to the Mineralogy of Nova Scotia, III. Borates and other Minerals in Anhydrite and Gypsum", Philosophical Magazine, pp.1-10, January 1868.
H. How, "Contributions to the Mineralogy of Nova Scotia, IV. Lignite - Semibituminous Coal - Cannel Coal - Turgite - Delessite - Fahlunite - Silicoborocalcite", Philosophical Magazine, pp.1-10, April 1869.
V.S. Papezik and C.C.K. Fong, "Howlite and Ulexite from the Carboniferous Gypsum and Anhydrite Beds in Western Newfoundland", Canadian Mineralogist, vol. 13, pp. 370-376, 1975.
Mineral Spectroscopy Server, California Institute of Technology (http://minerals.gps.caltech.edu/COLOR_Causes/Radiate/)
Salt in Nova Scotia, Nova Scotia Department of Natural Resources, Minerals and Energy Branch, Information Circular ME 33, 1992. (http://www.gov.ns.ca/natr/meb/ic/ic33.htm)
A Virtual Field Trip of the Landscapes of Nova Scotia, (http://www.gov.ns.ca/natr/meb/field/start.htm)

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