rocks

Most of the specimens featured here, unless they are fossils

Sapphires in the Rough

16 small irregularly shaped translucent stones ranging from colorless to blue.

Frederick, one of our members who is into gem cutting, brought in these rough sapphires from Montana at our last meeting. Check out his website http://missouririversapphire.co/ to see how beautiful they look when they are cut.

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Pyrite

An irregularly shaped chunk of pyrite with black and gold streaks throughout.

Photo by Stephanie Reed

At our August meeting, Martin showed off this unusual iron pyrite specimen from Colorado. The gold parts are pyrite and the black streaks contain small amounts of gold ore.

Big Al’s Big Rock Sale

Flyer for Big Al's Big Sale with list of tools for sale.

“Big Al” DeMaranville is having a rock and tool sale in Kansas City, Kansas so that he can move into a new home. On the first weekend, July 25-26 (8am-5pm), there will be rocks and lapidary/silversmithing equipment for sale. The second weekend, August 7-8 (8am-5pm), will have a house sale (and any tools that haven’t been sold yet).

The sale is now over. Thank you for your interest.

Canadian Rocks on Display

Tanya at Dans Le Lakehouse has a neat collection of minerals from Canada that she’s had since childhood. She wanted to enjoy her shiny pretty rocks, but they were stored in an opaque cardboard box. One day, she found this glass box that was the same size as the original box and lined it with felt. Then she arranged the specimens by color and hid the name and locality tags underneath the felt. I don’t know why she thinks this is nerdy. Now the rock collection sits on her husband’s desk, adding color to the room. Read her blog post for more details.

What a neat way to display a colorful set of minerals! I think the sodalite and the red jasper are the most eye-catching. Which mineral is your favorite?

Scepter Quartz

Article by Amir Chossrow Akhavan, http://www.quartzpage.de/gro_text.html

A scepter quartz is often defined as a quartz crystal that has a second generation crystal tip sitting on top of an older first generation crystal. The second generation tip typically becomes larger than the first generation tip, but might also become smaller. A scepter can be shifted sideways and does not need to be centered on the first generation tip.
However, there is a problem with a definition that is based on the idea of a second generation: scepters do not only occur as a second generation on an older crystal, they also form stacks of parallel grown crystals that developed at the same time, very often as skeleton quartz. Another difficult case are reverse scepters in which the scepter is smaller than the underlying tip. Here the smaller tip very often does not show any properties that clearly distinguish it from the rest of the crystal and that would justify calling it a second generation. Instead, the crystals often appear to have grown continuously into the reverse scepter or multiple scepter shape.

In all cases, the scepter develops from the already present crystal lattice of the crystal underneath. Thus, to be a scepter quartz, the “second generation” crystal’s a- and c-axes need to be oriented parallel to the respective axes of the “first generation” crystal; just one crystal on top of another doesn’t make it a scepter. Such a crystallographically well defined intergrowth of different minerals is called an epitaxy. In a sense, a scepter represents an epitaxy of quartz on quartz, and because it is the same mineral, it is sometimes called an autotaxy.

Scepters are quite common in certain geological environments. Amethyst from alpine-type fissures in igneous and highly metamorphosed rocks usually occurs as scepters on top of colorless or smoky crystals (not only in the Alps, but for example also in southern Norway or northern Greece). Here, the amethyst generation grew at lower temperatures than the first generation quartz. The same growth form can be observed in pegmatites and miaroles in igneous rocks (for example, amethyst scepters from the Brandberg, Namibia, or from pegmatites in Minas Gerais, Brazil).

Scepters, or to be precise, the “second generation” part of a scepter quartz that defines it, commonly have a number of morphological properties:

  • Scepters are commonly of normal habit and are never tapered. The underlying “first generation” crystal may show a Tessin habit, but the scepter on it will not.
  • Scepters tend to assume a short prismatic habit. An apparent exception are reverse scepters and the normal scepters associated with them, which may occur as elongated extensions of a “first generation” crystal, but then in the shape of multiple stacked scepters.
  • Many scepters show only a weak striation on their prism faces, sometimes it is even missing.
  • Scepters do not show split growth patterns.
  • Scepters rarely show trigonal habits with very small or missing z-faces. An exception are reverse scepters and the normal scepters associated with them.
  • Scepters are often associated with skeleton growth forms (skeleton or window quartz).
  • Scepters commonly show a color, color distribution, diapheny and surface pattern that is markedly different from the underlying “first generation” crystal. Often they are more colorful and transparent. Amethyst scepters are very common, smoky quartz scepters -often with uneven color distribution- are common. An exception are reverse scepters and the normal scepters associated with them which seem to either not differ from the “first generation” or show gradual transitions.
  • Summarizing the exceptions above: Reverse scepters and the normal scepters associated with them seem to have a different set of properties.

Formation

One theory is that a scepter forms when crystal growth is interrupted and parts of the crystal are covered with some material that inhibits further growth. The growth inhibiting material might be only present as a very thin layer and invisible. The very tip of the crystal or the entire rhombohedral faces remain free of that material, and should the conditions change again, the crystal continues to grow from the tip.

One of the problems with that theory is that you would expect to see a larger number of “double”, “triple” or “quadruple scepters”, specimen in which the growth had been interrupted several times and in which scepters with slowly changing habits are stacked. In nature, however, you see a strong dominance of “simple” scepters that consist of just a prism with “a single head”. If you see multiple scepters, then often alongside simple scepters, although multiple changes in the environment should have affected the morphology of all of them equally.

Another problem is that you would not expect to see a fully-grown scepter that encloses the former tip like an onion if the crystal simply started growing from a single point on the surface of the tip. Such a crystal would finally grow into an elongated crystal and would at best assume the shape of a reverse scepter.

As I’ve mentioned, amethyst from igneous and metamorphic rock locations all over the world predominantly occurs as scepters. Even if you just take Alpine locations, it is hard to imagine that the environmental conditions in all those locations have undergone a single sudden change that led to a temporary growth inhibition on the crystals, followed by a very distinctive growth pattern, the formation of scepters.

The internal structure of scepters from Alpine-type fissures (and of scepters in general) is perhaps always lamellar, as opposed to the macromosaic structure of many quartz crystals from Alpine-type fissures. Quartz crystals with a macromosaic structure may carry a scepter, but the scepter will then show lamellar structure.

Fossil Sweet Gum

A big slab of petrified wood that is green

Photo by Stephanie Reed

This is a cross-section of a fossilized sweet gum tree from the Hampton Butte in Crook County, Oregon. We saw it at the Rice Museum in Hillsboro, Oregon where it is in the petrified wood room. I hardly ever see petrified wood that is green like this; usually it’s red, orange, or brown. Anybody know what makes it green?

Corundum

Corundum (Al2O3) is a hematite group mineral that has trigonal crystals. It is found all over the world and can be many different colors including blue, red, pink, yellow, gray, and colorless. These corundum crystals are from the Cascade Canyon in San Bernadino, California. They may not look familiar to you, but corundum has some famous relatives. A gem-quality corundum that is red (Cr-bearing) is known as ruby, and a gem-quality corundum that is blue (Fe- and Ti-bearing) is known as sapphire.

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.