Wisconsin Moonstone

When you think of rockhounding in Wisconsin, you probably think of Lake Superior agates. But did you know that Wisconsin also has moonstone? Read this article from the MWF January 2015 newsletter to find out more.

Anorthoclase moonstone from Wisconsin.

Image from Bill Schoenfuss and Moonlight photography.

WISCONSIN’S MOONSTONE

by Dr. William S. Cordua

Emeritus professor of Geology

University of Wisconsin – River Falls

Imagine an October full moon in Wisconsin glowing ghostly blue to yellow as it seems to float over the newly harvested farm fields. Or is this captured in the rock? In Wisconsin’s own moonstone?

Wisconsin moonstone has been known for decades, but only recently have skilled lapidarists learned to work it to bring out its full beauty. This find surprises non-residents, who at generally associate Wisconsin gemstones with Lake Superior agates and nothing else. What is this material? How did it form? What causes its optical effect?

The moonstone localities are on private land in central Wisconsin, not far from Wausau in Marathon County. The mineral is a type of feldspar known as anorthoclase. This formed as a rock-forming mineral within the Wausau Igneous Complex, a series of plutons intruded between 1.52-1.48 billion years ago. There are at least 4 major intrusive pulses within the complex.

The anorthoclase is in the Stettin pluton, the earliest, least silicic and most alkalic of the plutons of the Wausau complex. This body is complexly zoned, largely circular in outcrop and has a diameter of about 4 miles. It is mostly made of syenite, an igneous rock resembling granite, but lower in silica and higher in alkali elements such as potassium and sodium. As such, it lacks quartz, but does contain a lot of alkali feldspar. Further complicating the geology is the intrusion of later pegmatite dikes. Some especially silica-poor varieties sport such odd minerals as nepheline, sodalite, fayalite, and sodium rich amphiboles and pyroxenes. Zircon, thorium, and various rare earth element minerals can be found in this pluton. Large prismatic crystals of arfvedsonite and nice green radiating groups of aegirine (acmite) crystals have been collected for years from these rocks. It is also the pegmatite dikes that contain the anorthoclase showing the moonstone effect.

The moonstone has been found in small pits and quarries and also in farm fields where masses weather out and get frost-heaved to the surface. The weathered masses of coarse cleavable feldspar may at first not look too interesting, but at the right angle the moonstone effect can be seen. The feldspar has two change and bounding capacity, so fit readily in the same niches in the feldspar. But sodium and potassium aren’t enough alike. If the feldspar cools down slowly, to below 400 degrees C, the feldspar structure contracts in size, and sodium and potassium are no longer good interchangeable fits. The homogenous anorthoclase splits on a fine scale into intergrown potassium feldspar and albite. Sometimes the bands of alternating minerals are coarse enough to see. Other times they are microscopic. If they are just the right size and spacing, they scatter the light that penetrates the various layers in the mineral – producing the moonstone effect, or schiller. The only anorthoclase that is truly not a mixture is that which cools very rapidly, such as in lava flows, so the separation cannot occur, and the mineral is frozen into its high temperature form. The material at Wausau cooled slowly, so isn’t, strictly speaking, anorthoclase anymore, but an exsolved mixture.

The crystalline structure controls the orientation of these exsolution bands, hence the effect is seen better on some surfaces (the {010} cleavage for example) than at others. This is one reason why shaping the rough stone takes such skill. Other challenges are the weathered nature of some of the stone, and exploiting the cleavage directions inherent in the feldspar.

Polished moonstone fragment several centimeters long showing the moonstone effect.

Image from Bill Schoenfuss and Moonlight photography.

The master of processing these stones is Bill Schoenfuss of Wausau, Wisconsin. Bill often exhibits and sells his beautifully prepared moonstone at shows in the upper midwest. He can be contacted at schoenfuss

Moonstone has been prized as a gem since antiquity, often characterized as being like solidified moonbeams. The Greeks and Romans both related the gem to their moon gods and goddesses. The American Gem Society considers moonstone an alternate birthstone for June.

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