Canadian Rockhound: Weird and Wonderful Quartz

Weird and Wonderful Quartz in
Septarian Nodules from Alberta's
Dinosaur Country
By Dr. Mike Menzies

Since moving to Alberta twenty years ago, about the same time as I discovered mineral collecting, the local sedimentary geology has proved to be a mixed blessing. It has indirectly paid my salary (as an engineer in Alberta's oil patch), but it offers a real dearth of mineral collecting localities. Thus I was excited (though a little skeptical) to learn of a nearby quartz collecting locality. It is a three-hour drive northeast from Calgary to Donalda, a small village that is better known for its lamp museum and as a stop for summer steam locomotive excursions.

My first collecting specialty was quartz, for which I've retained a soft spot, and I was not disappointed in this new locality. Several trips over the last three years have produced a wide variety of interesting and unusual quartz crystals. These are found in abundant ironstone nodules that weather out of steep slopes along the coulees of Meeting Creek and the Battle River in the vicinity of Donalda.

The locality is in the Alberta Badlands which are renowned for dinosaur fossils. These fossils are particularly prevalent south of Donalda in the Drumheller area which is home to the well-known Tyrrell Museum, but they have been found in the Meeting Creek coulee. A common geological horizon, the Upper Cretaceous Edmonton Group, hosts both the fossils and the quartz crystals. The Edmonton Group consists of up to around 1,200 feet (350 m) of sediments that were deposited in fresh to brackish water while shallow seas were receding and the land to the west continued its uplift to form the Rocky Mountains. This sedimentary sequence is made of sandstones, shales, bentonite clays, ironstones, and economic deposits of coal.

The dark red-brown clay ironstone forms ledges and nodules that stand out very clearly within the thick layers of light-colored bentonite clay. These nodules range from cm size to 12 inches (30 cm) or more. Many are solid, locally with inclusions of petrified wood. Closer examination in some areas shows internal shrinkage cracks which, when filled with honey-colored calcite, show the typical internal structure of septarian nodules. Less commonly the filling is quartz, which shows variable degrees of crystallization depending on the available empty space remaining within the typically narrow cracks. There is considerable local variation, typically over short distances, in both the extent of fracture formation and the fracture-filling minerals that include calcite, chalcedony, and crystallized quartz.

Since I hadn't associated septarian nodules with quartz, I made a brief literature survey. This confirmed that septarian nodules or concretions, which are relatively common worldwide in sedimentary deposits, typically contain carbonates, sulfates, less commonly sulfides, and even organic compounds such as petroleum. Quartz as a fracture filling, especially in good crystals, appears to be rare, and the variety of unusual crystal forms makes this occurrence all the more fascinating.

The best collecting exposures are in the rapidly eroding, steep sides of the coulees and the isolated hillocks within them. According to local farmers, crystals may be found anywhere there is ironstone, within an approximate triangle formed between Donalda, Meeting Creek and Rosalind. The occurrence of quartz crystals is well known to locals, and there has been heavy collecting from Red Deer and Edmonton clubs in some areas, but in other locations the abundance of surface material suggests little or no collecting. The Edmonton Group is present across a much wider area, but collecting opportunities seem to be limited to areas of good exposure along favorable sections of coulees.

Crystals are found by breaking open nodules, or more productively by looking for nodules disintegrating in place or material completely weathered out on surface. This is some of the easier collecting I've ever done!

In contrast to the calcite and chalcedony, quartz is generally poorly attached to its ironstone matrix. It shows some very unusual forms spanning a fascinating range of crystallization that may be present to some degree on any single specimen. All quartz appears to represent a single generation, although there does seem to be some progression along the sequence suggested below.

Descriptions of these different forms follow, concluding with opal, petrified wood, and calcite and an unknown species (which may be siderite) which are included for completeness.

Chalcedony is the predominant quartz fracture filling in limited areas. It also forms the matrix for pseudocubic crystals of quartz and rarer drusy quartz. Elsewhere minor chalcedony is commonly associated within either prismatic quartz crystals or petrified wood (typically forming on ends of sections of wood). One specimen of petrified wood shows a small area of white and grey banded agate. All chalcedony appears to fluoresce under ultraviolet light, more strongly under long-wave, and with moderate intensity. The color is typically medium yellow, less commonly white.

"Pseudocubic" crystals are the rarest form of quartz. They form sharp cubes ranging from almost transparent single crystals to more commonly opaque, intergrown crystal masses. Crystal transparency and luster resembles that of the chalcedony which either forms the matrix or is in close association. This link with chalcedony is also reflected in the fluorescent behavior. As with chalcedony, long-wave ultraviolet produces the strongest response, with pseudocubic quartz showing much weaker fluorescence, typically dull orange. My largest crystal to date is over 1 cm in size, but crystals even approaching this size were found only in two very small areas. These crystals are also associated with petrified wood, less commonly with normal quartz crystals. Micro crystals on petrified wood are uncommon but appear to be somewhat more widely distributed. Two possibilities are suggested to explain this morphology: either a pseudocubic habit for quartz or quartz replacing an earlier cubic mineral.

Pseudocubic form typically results from development of only one of the two sets of rhombohedral termination faces to the exclusion of the prism and other crystal faces. According to King and Foord, such pseudocubic quartz is quite common worldwide. In describing micro crystals from further south near Drumheller, Sabina supports the pseudocubic argument. Similar micro crystals are also reported by Allan Ingelson (personal communication) who provided me with a specimen from still further south near Manyberries, and also by Ty Ballacko (personal communication) in describing crystals on carbonized (but not petrified) wood from the Edmonton area. These Alberta localities all represent comparable geological environments. The crystals are, however, rather different from those described by King and Foord in several respects, including their chalcedony-like texture, lack of any visible modifying faces and typical form consisting of multiple, intergrown but otherwise simple pseudo cubes.

In fact, the texture of the crystals from Donalda argues more strongly for a replacement origin, although there is lack of other supporting evidence. Such evidence might include 1) any earlier (or for that matter even associated) cubic mineral, 2) internal structure within the crystals, and 3) a second generation of chalcedony that would be required to form replacements on the typical chalcedony matrix.

Drusy quartz, which forms on chalcedony (or in one case on common opal), is rather uncommon.

Crystals of prismatic habit. The other forms of crystallized quartz can all be generally classified as "prismatic," though crystals are typically highly distorted and unusual. Well-terminated quartz crystals are uncommon due to the lack of free space for crystallization, and single crystals of simple habit are decidedly uncommon! The broad but typically narrow shrinkage cracks more commonly appear to encourage formation of irregularly shaped parallel-growth aggregates forming plates, sprays, and even complete rosettes. Some of these may even be curved to follow the shape of the cracks. Crystal faces commonly show "flow patterns," and regular or reverse sceptering as well as complex multiple terminations are very common. Other rarer forms, that include cavernous crystals with multiple parallel-growth terminations, complete a veritable catalogue of fascinating features. In this area, it seems that the unusual is the norm rather than the exception!

Sizes for crystal groups and plates reach 4 inches (10 cm). Cavernous crystals show face widths to 3/4 inch (2 cm). Individual crystals, which are up to 11/2 inches (4 cm) in length, are more typically in the micro to mm size range, with forms typically sufficiently complex to require 10x magnification for the fullest appreciation. The best crystals are colorless, lustrous and gemmy (especially at the tips, rarely throughout the crystal), but many are milky with dull luster. A few crystals of very light amethystine color have also been found in one location (Barry Murphy, personal communication), but unfortunately there hasn't been even a hint of color in the material I've collected to date.

Common opal has been identified from a single specimen as a small white patch which forms the matrix for drusy quartz. The specimen, which has a core of petrified wood, also shows prismatic quartz crystals.

Petrified wood is locally common, typically associated with chalcedony (which may have an overgrowth of pseudocubic crystals), rarely with quartz crystals of prismatic habit.

Calcite is commonly a granular, honey-colored mineral that completely fills individual fractures, more commonly the entire void space within a nodule, but crystals to 5 mm form in uncommon open spaces in calcite-filled nodules. Micro crystals are relatively rare in association with well-crystallized quartz, and depending on location, calcite may form earlier or later than quartz. Sharp, doubly-terminated micro scalenohedrons occur on pseudocubic quartz from one location.

An unknown species that may be siderite has been found at a single location. Crude, chocolate-brown micro crystals, some of which are disc-shaped, occur on pseudocubic quartz.

Further collecting may uncover other, perhaps still more unusual forms of quartz. Nature is also busily creating new exposure by slope erosion that is geologically extremely rapid at up to around 3/4 inch/year (2 cm/yr). There is much ground still to cover, but land in the area is generally privately owned so that collectors should seek permission from local farmers. Collecting is not recommended in wet conditions or after recent rain which can make the clay slopes very slick.

Specimen cleaning is relatively easy. Soak overnight before using a water jet (a hose connection with a very small hole in a plate soldered across the outlet works well) to remove most of the dirt and clays, then use a dental pick to remove scraps of matrix, and mild air abrasive to remove any obstinate clay coating. Some quartz may benefit from acid treatment to remove residual iron staining, but it is best to use hydrochloric rather than oxalic since invariably there seems to be calcite present. It is best to avoid acid treatment of matrix specimens since this may turn the ironstone a rather unattractive rust color (though the original color may be restored with further air abrasive).

I would be most interested to hear from other collectors of other occurrences of both quartz crystals in septarian nodules and pseudocubic quartz associated with chalcedony.

– A follow-up article is available online. See "Weird and Wonderful Quartz - An Explanation for the Formation of Pseudocubic Quartz," Canadian Rockhound, (Winter/Spring 1999).

Acknowledgements

My thanks to Barry Murphy, and also Joan and Luc Guillemette, who shared with me their knowledge of the locality. Allan Ingelson provided the lead on the occurrence of pseudocubic quartz, and Ty Ballacko described another similar occurrence. Thanks also to a number of local residents for their hospitality and permission to collect on their land.

References

Allan, J.A., and Sanderson, J.O.G. Geology of Red Deer and Rosebud sheets, Alberta (1945) Research Council of Alberta, Report No. 13, 109 p.
Beaty, C.B. The landscapes of southern Alberta (1975) University of Lethbridge, 95 p.
Donalda's roots and branches (1980) U-Go Weavers History Book Committee, Donalda, Alberta, 554 p.
Ingelson, A. The Edmonton Formation (1978) Canadian Rockhound 22(4):14-19.
King, V.T., and Foord, E.E. Mineralogy of Maine (1994) Maine Geological Survey, p. 292.
Rutherford, R.L. Geological Survey of Canada Map 503A (Stettler, Alberta) (1939).
The land before us: the making of ancient Alberta (1994) Royal Tyrrell Museum, Red Deer College Press, Red Deer, Alberta, 96 p.
Sabina, A. Some new mineral occurrences in Canada. (1978) Geological Survey of Canada, Paper 78-1A, p. 253-258.
Shaub, B.M. Genesis of agates, geodes, septaria and other concretions of sedimentary origin - Part II (1980) Lapidary Journal 34(7):860-896.
Weaver, R. Septarian nodule (1983) Lapidary Journal 37(1):1684-1686.

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The preceding article was first published in the December 1997 issue (Vol. 13, No. 12) of Mineral News. Reprinted in the Canadian Rockhound with permission from the author.