This article was originally publiashed in the Midwest Federation News Letter

Winter is a great time to get out the U.V. light and look at your mineral collection. Luminescent (fluorescent and phosphorescent) minerals can pop up anywhere. My state of Wisconsin is a not state not widely known for its fluorescent minerals, but there are some to find, and likely more waiting for somebody to turn their U.V. lamp on them. Here is a summary of what I know of Wisconsin luminescent minerals, followed by a cautionary tale. In the following, SW refers to short wave U.V. light, MW refers to medium wave U.V. light , and L.W. refers to long wave U.V. light.

Calcite is found many places in Wisconsin and often shows luminescence. Calcite from cavities in the state’s Ordovician carbonate rocks, notably the Prairie du Chien Group, usually fluoresce yellow or blue with blue phosphorescence under SW, MW and LW. These calcites can be found as small crystals in vugs, and as travertine or other speleothems. I found some of the best material in quarries in the area across the Mississippi from Winona, Minnesota. Here some of the dolostones in the Prairie du Chien Group have cavities filled with calcite cleavage rhombs up to 5 inches (12 cm) across. These fluoresce blue with purple-red regions that phosphoresce bright blue. SW MW and LW. I’m most familiar with outcrops in the western part of the state (Hudson south along the Mississippi River to the Illinois line). Collectors in central and eastern Wisconsin should check their calcites for luminescence.

While we are focused on the Prairie du Chien, note that the dolomite making up most of the rock generally fluoresces yellow SW and MW. The fluorescence is not bright, but it is attractive when associated with blue fluorescing calcite.

Quartz in its massive form (not chalcedony) usually does not fluoresce. However some of the quartz crystal druses and cherty bands from the Ordovician Prairie du Chien Group will fluoresce faintly yellow, orange or green. Such fluorescing quartz has turned up in quarries and road cuts near Hudson, Spring Valley, Nelson and Green Lake, so there are doubtlessly other occurrences.

The lovely scepter crystals of calcite from the Upper Mississippi Lead Zinc region of Grant, Lafayette and Iowa Counties will fluoresce red to yellow SW and MW, and also phosphoresce. These are not the only fluorescing minerals from Wisconsin’s zinc-lead district. Earthy hydrozincite found as alteration on sphalerite on old mine dumps fluoresces bright blue SW. Smithsonite, found on the dumps of the Little Giant Mine near Shullsburg fluoresces blue, yellow and red SW and MW.

Calcite and dolomite were locally found in the now-closed Flambeau Copper Mine near Ladysmith. Some of the dolomite fluoresces yellow SW and MW. Some of the calcite from the mine fluoresces red to orange SW and MW but yellow LW.

Another well-known Wisconsin material, the pinkish manganese-bearing calcite from the Montreal Mine near Hurley, is also well-known for its red fluorescence SW, MW and LW. White calcite from this mine will fluoresce yellow SW and MW.

Calcite in veins cutting metamorphic rocks in the center of the state, such as near Wisconsin Rapids, Neillsville and Thorp often fluoresce red SW and MW. White calcite found in vugs and veins associated with the Keweenawan volcanic rocks in northern Wisconsin (such as exposed near Superior and near Hurley) also will fluoresce red SW and MW.

The granitic rocks of the Wausau, Stettin and Marshfield area contain several fluorescent minerals. The potassium feldspar that is a major component of these rocks will generally fluoresce cherry red SW. This includes the lovely “moonstone” from the Stettin area. These granitic rocks can also contain zircons that fluoresce yellow SW and MW. Fluorite found in the granitic rocks of the Wausau and Cary Mound area can fluoresce yellow, blue or green, sometimes in zones, under SW, with a blue phosphorescence. Some of the granitic rocks have crusts of hyalite opal that fluoresce green SW MW and LW. The hyalite is white to nearly clear, so can be easily overlooked until you turn on the UV light.

Yellow fluorite from the old quarry near Sturgeon Bay is brightly fluorescent and phosphorescent yellow-white in SW, MW and LW. Strontianite crystal clusters on dolomite from Mackville fluoresce blue-white SW, MW and LW. Finally, although not a mineral, slag from the old iron furnace in Spring Valley, Wisconsin fluoresces orange SW and MW.

I have a good rock hound buddy who just got a new SW UV light with a battery pack. He asked me what would happen if he took it into one of our western Wisconsin gravel pits some night. Well, who knows? Just about anything is possible in that glacially blended gravel. One night he went into a pit owned by a friend of his. He was happily using his light, supplemented by occasional use of a flashlight to avoid tripping over stray boulders. Suddenly he was framed in a strong spotlight. He began cursing whoever was messing up his night vision, when he also saw some flashing red lights. It turns out that a county policeman was suspicious of these odd light “signals” down in the pit after dark. The policeman was not impressed by my buddy’s UV light and called the pit owner. Since the owner was a friend, my embarrassed rock hound buddy was let go, although his friend laughed a lot at him. This emphasizes the importance of having permission before you go collect on somebody ’s property. But – did my friend find anything? Yes, he got a nice cobble of granite with yellow fluorescing zircons thickly scattered in red fluorescing potassium feldspar. You can never tell what you might find!

If others have knowledge of other Wisconsin fluorescent minerals, please let me know.