Porcelaneous Datolite of the Keweenaw Peninsula, Michigan
By: Paul T. Brandes

Datolite. When one thinks of this mineral, they usually envision the beautiful green crystals of Dal’nagorsk, Russia or perhaps Mount Sainte-Hilaire, Quebec. However, there is another, lesser known variety of datolite that is perhaps rarer than the crystallized form; this would be the porcelaneous variety that is found on Michigan’s Keweenaw Peninsula. The following is a brief description of what a datolite is and its mode of occurrence in Michigan, what gives the datolites of Michigan their striking colouration, historical information, and early collecting and beyond.



Introduction

Datolite is classified as a calcium borosilicate hydroxide with the chemical formula of Ca2B2Si2O8 (OH)2. The most common form is as monoclinic crystals that are glassy in appearance and complex in habit. In Michigan, datolite also forms in crystals; however, the most common form on the Keweenaw Peninsula is in porcelaneous masses that can range from the size of a pencil eraser to as big as a bowling ball and can weigh as much as 150 pounds.

In the Lake Superior district, datolite forms along side the great native copper deposits contained with the Portage Lake Volcanics (PLV). Datolite occurs in fissure veins and brecciated basalt flowtops. Interestingly, datolite is not found in the conglomerate interflow deposits of the PLV. The porcelaneous datolite occurs in several modes; as a fracture filling to produce narrow veins, as cement in the brecciated flowtops, and as semi-round nodules resembling a head of cauliflower. The datolite has a texture that ranges from a dense porcelain-like appearance to a more sugary, granular material and can be translucent to opaque. The one feature of the Michigan datolite that draws people to it, however, is its phenomenal range of colour, which can cover the entire spectrum range.

Historical Information

Dr. C.T. Jackson is credited with the first written description of datolite, which he wrote in his diary as “datholite”. In an entry from 1849, Dr. Jackson wrote in his diary that “datholite may prove of economical importance as either a flux for copper ores or as a material suitable for the manufacture of borax”. Dr. Jackson also made mention of a native copper deposit on Isle Royale that was so rich in datolite they named the location the Datolite Mine.

In 1859, J.D. Whitney wrote about fine datolite crystals being found on Isle Royale and also at Keweenaw Point. Whitney also described the first reported occurrence of a porcelaneous mass of datolite at the Minesota Mine in Ontonagon County. He described the mass as having the appearance of a “rusty cannonball” and that upon breaking the nodule open discovered that the mineral inside was “quite compact, opaque, perfectly white, and resembling pure marble”.

It wasn’t until 1895 when Osann wrote of occurrences of crystallized datolite at several of the mines on the Keweenaw including the Clark, Osceola, and Copper Falls. In the paper was also mention of the very colourful datolite nodules with copper inclusions that were being discovered in the Pewabic Lode.

The first comprehensive work on the native copper deposits of the Lake Superior District was completed in 1929 by Butler and Burbank. United States Geological Survey Professional Paper No. 144 listed datolite as being noted in fissure veins and amygdaloidal lodes of the District but again, not from the conglomerate lodes. In the Paper was also a paragenetic sequence that puts the deposition of datolite in the District as an intermediate to late stage mineral when compared to native copper deposition.

Colourization of Datolite Nodules

One of the first things that collectors and other people in general notice when they first observe a datolite nodule from Michigan is the wide range of colours and patterns the nodules can have. The most common colour of Michigan datolite is bone white, while the most valuable and desirable colour is yellow, and the rarest of colours appears to be the purple variety. The earliest writings about Michigan datolite make mention of native copper flakes scattered in the nodules, but give very little attention to colouration or its causes. It wasn’t until 1978 when Michigan Technological University in Houghton began research to determine what the causes of colouration were in datolite that an official scientific study was conducted. Since this time, several other studies have been conducted to determine the possible causes of colouration in specific datolites. From this research, it has been determined that pure datolite occurring in nodular form is white. A pink/red to yellow colour was most likely the result of iron oxide (Fe2O3) in varying crystal size of the iron, while chalcotricite was the likely cause of colouration for orange and red datolites. Oxidation of copper minerals during supergene enrichment, which can result in malachite and chrysocolla, give the nodule a green or blue colour. Other minerals that give datolites their colour include (but not limited to) tenorite, bornite, cuprite, and rarely azurite.

Early Collecting and Beyond

In the early days of mining on the Keweenaw Peninsula, employees of mining companies began collecting minerals such as copper and silver to either display in their homes or sell them as “collector specimens” to buyers all over the United States. It is believed that datolite was not considered a collector mineral at this time, partly because their inner beauty could not be revealed unless sawed into with specialized equipment which, at this time period, was not readily available. In later years, other mining employees and specific mineral collectors began to amass huge collections, although these focused mostly on crystallized specimens such as copper, silver, and calcite. It wasn’t until mineral collectors such as J.T. Reeder and Dr. L.L. Hubbard (who were also mining employees) acquired outstanding yellow and salmon coloured datolites from the Pewabic Lode that datolite became a true “collector mineral” in the Keweenaw.



Throughout the late 1800’s and into the early 1900’s, collectors had access to underground material through miners and to some extent, even mining captains. However, by about 1920, all of the great fissure mines had closed and thus the supply of “fresh” datolites had ended. The amygdaloidal lodes were still producing copper and a few collectors were allowed access to the poor rock piles to search for specimens. By the 1940’s and 1950’s collectors began to search the piles in all three counties (Keweenaw, Houghton, and Ontonagon) to collect datolites and also classify nodules so that a particular colour or pattern in a given datolite nodule could be associated to a specific mine or lode. By the 1960’s, datolites began gaining the eye of more collectors throughout the midwest and central United States, with more and more people coming to the Keweenaw in search of the colourful datolite. During this time, several collectors in the Keweenaw had amassed very sizable and important collections of datolites from throughout the Peninsula. By 1970, the days of going to a poor rock pile and easily collecting a nodule of datolite for yourself were done. From this point on to today, datolite collecting has been a hit-and-miss proposition at best. Occasionally, a new and exciting find of a datolite “nest” is reported, but it is usually short lived as collectors rush to strip every last nodule from a location. Another common practice is to wait for a particular county to setup crushing equipment at a poor rock pile and then search the area of the pile where material has been removed. This again has provided a few new and exciting finds, but one has to be quick to collect before it is stripped clean.

Of the over 300 known mines and exploration prospects on the Keweenaw Peninsula, only about eighty have produced datolite nodules while less than half of that number have produced collector quality nodules. According to experienced collectors on the Keweenaw Peninsula, three mines have produced enough quality datolites to be considered as classic localities. These are the Delaware Mine in the northern section, the mines of the Pewabic Lode in the central section, and the Caledonia Mine in the southern section. Each of these mines are famous for producing nodules of exceptional colour, size, and that are unfractured in nature. Datolites from these locations also command some of the highest values of any datolites found on the Keweenaw Peninsula.

Today, as more and more of the poor rock piles are being removed for various projects on the Keweenaw Peninsula, datolite nodules are getting more difficult to find every year. Sometime in the not too distant future, when either all the piles are removed, or the remaining piles are sold to private individuals that do not share the same interest of collecting specimens, the practice of going to a poor rock pile and finding your own datolite nodule will just be a romantic memory.



Bibliography

Butler, B.S., and Burbank, W.S., 1929, The Copper Deposits of Michigan. U.S. Geological Survey Professional Paper 144.

Heinrich, E.W., 2004, Mineralogy of Michigan, 2nd ed., revised and updated by Robinson, G.W.: Houghton, Michigan Technological University, 251 p.

Jackson, C.T., 1849, On the Structure of Keweenaw Point, in American Association for the Advancement of Science Proceedings. Vol. 2. p. 288-301.

Kilpela, T., 1993, The Hard Rock Mining Era in the Copper Country. Privately Published.

Osann, A., 1895, Uber Datolith vom Lake Superior und die ihn begleitenden Mineralien, in Zeitschrift fur Krystallographie und Mineralogie. Vol. 8. p. 543-555.

Rosemeyer, T. 2003, The Occurrence of Porcelaneous Datolite in Michigan’s Lake Superior Copper District: Part 1 – Northern Keweenaw County and Isle Royale National Park. Rocks and Minerals. Vol. 78. p. 170-188.

Rosemeyer, T. 2005, The Occurrence of Porcelaneous Datolite in Michigan’s Lake Superior Copper District: Part 2 – Southern Keweenaw, Houghton, and Ontonagon Counties, Michigan. Rocks and Minerals. Vol. 80. p. 154-177.

Whitney, J.D., 1859, Notice of New Localities, and Interesting Varieties of Minerals in the Lake Superior Region. American Journal of Science. Vol. 28. p. 8-20.

Wilson, M.L., and Dyl, S.J., 1992, The Michigan Copper Country. Mineralogical Record. Vol. 23. p. 1-76.