Sutton Museum

A rock museum in Kansas City, on the UMKC campus

How Amethyst Cathedrals are Formed

purple amethyst cathedral in a museum with other minerals

Amethyst cathedral at the Sutton Museum. Photo by Stephanie Reed

Article by Dr. Bill Cordua, University of Wisconsin-River Falls

Have you ever been to a show and seen enormous amethyst geodes or crystals 3-5 feet or more in height? The tubular geodes are lined with deep purple gemmy amethyst crystals. How do such wonders form?

These excellent geodes come from a region along the Brazil-Uruguay border. The genesis of deposits on the Brazil side of the border has recently been extensively researched by an international team of geochemists lead by H. Albert Gilg of Techniche University Munchen in Germany (Gilg, et. al., 2003). The geodes are mined from several lava flows belonging to the Parana Continental Flood Basalt Province. This was one of the largest outpourings of basalt lava known. An estimated 800,000 cubic kilometers of lava extruded over an 11 million year time span. For comparison, this would be enough to cover Minnesota with a pile of basalt lava over 2 miles high. The lava outburst occurred as part of the opening of the South Atlantic Ocean during Cretaceous time about 130 million years ago. Of all these flows, however, only a few are known to host amethyst cathedral geodes.

Gilg et al. proposed a 2-stage model for their formation. In the first stage the large hollows form. This was caused as volcanic gases were released from certain lavas as they cooled. Not every lava has enough dissolved gas to form such big openings. As gas bubbles emerged from the congealing lava (much as bubbles emerge when beer or soda pop is poured) they coalesced as they rose. The lava was cooling fast too, and soon became so thick and sticky that bubbles quite rising and were trapped. The bulbous to tubular shapes thus point towards the top of the flow, a fact easily seen when the geodes are in place in the mines. These cavities, though, were empty of crystals.

The second stage was the formation of the amethyst, plus celadonite, calcite and gypsum fillings. An important clue to this event is the presence of small gas and liquid bubbles (called fluid inclusions) trapped within these minerals. These are samples of the mineral-forming liquids caught as the crystals grew. Fluid inclusions are treasure troves of information when studied with sophisticated instruments. Analyses of the fluid inclusions in the amethyst, calcite and gypsum show them to be filled with slightly salty water. This water had a temperature of no more than 100 degrees C, and possible less than 50 degrees C, during mineral formation. These cannot be fluids related to the magma that formed the lavas.

What was the source of these fluids? An amazing story unfolds from the radiometric dating of the minerals. The basalts formed about 130 million years ago, but the green celadonite, which makes up the rinds of the geodes, formed about 70 million years ago. For 60 million years these enormous cavities sat empty of crystals. Trace element data from the fluid inclusions gives another important clue to the source of the mineral-forming fluid. Below the lavas is a large aquifer (the Botucatu aquifer) filled with ground water that closely resembles the fluid inclusion liquids. Uplift and tilting of the area about 70 million years ago would force water out of the aquifer into the porous areas of the overlying lava. In the lava flow these waters would have found volcanic glass. Glass breaks down over geologic time and makes silica and other chemicals available in a form that is readily soluble in water soaking through the rocks. The water carried these chemicals into the cavities, where the amethyst and other minerals grew due to cooling and pressure release.

The special combination of geologic circumstances, unfolding over millions of years, is not often duplicated. Understanding the process gives geologist tools to prospect more efficiently for these wonders.

Gilg, H. et. al, 2003, “Genesis of amethyst geodes in basaltic rocks of the Serra Geral Formation (Ametista do Sul, Rio Grande do Sul, Brazil): a fluid inclusion, REE, oxygen, carbon, and Sr isotope study on basalt, quartz and calcite” Mineralium Deposita vol. 38, p. 1009-1025.

The Glacial Drifter 08/2011, The Gemrock 06/2015

Pyritized Ammonite

A small ammonite fossil with gold sparkles of pyrite

Photo by Stephanie Reed

Here is a pyritized ammonite (Cosmoceras spinosum) from the Jurassic period, found in Michaelov, Russia. The specimen was at the Sutton Museum. How do pyritized ammonites form? I asked the Internet and here is what I found:

Pyrite or “Fools Gold” is an iron sulphide that occasionally – under unique geochemical conditions – covers or replaces prehistoric creatures and plants, transforming them into incredible fossils with a gold-like lustre.

Pyritized fossils tell us a lot about the past environments of our planet. Research indicates that prehistoric animals that become pyritized, such as trilobites and ammonites, were rapidly buried under ocean sediments that were low in organic matter. In this case there would not be a lot of decaying material present. Another important condition was anaerobic seawater – the water was low in dissolved oxygen.

For the trilobites with soft body parts, rapid burial meant there was very little decay of the creature before the fossilization process began. One of the final conditions for pyritization to occur is to have large numbers of sulphate reducing bacteria (they live in oxygen deficient water) and a high concentration of reactive iron. The bacteria change the sulphates into sulphides which can then diffuse with the iron into the trilobite or other organisms forming our spectacular fossils.


A Great Geology Book

Rocks and Fossils of the Central United States with Special Emphasis on the Greater Kansas City Area by Richard Gentile

The front cover of Richard Gentile's book, Rocks and Fossils of the Central United States with Special Emphasis on the Greater Kansas City Area

Front cover

The back cover of Richard Gentile's book, Rocks and Fossils of the Central United States with Special Emphasis on the Greater Kansas City Area

Back cover

Review by David Reed:

This book is great! It has beautiful pictures of the fossils that can be found in Kansas City and clear stratographic sections explaining the geology of the area. It also shows locations for picking up the fossils. Everything you might wish to know about Kansas City is in this book. Well worth the money and you can ONLY get it at UMKC (Amazon doesn’t have it). We purchased one when we visited the Sutton Museum.


A rock that looks like a cluster of small gray bits of shiny steel with spots of red rust.

Photo by Stephanie Reed

Hematite (Fe2O3) is a type of iron ore, which means that it contains iron which can be smelted out and used. It has a distinctive red color which means it can be used as red pigment, and is why hematite is sometimes called bloodstone. This is the main way you can identify hematite: it looks silver, but it produces a red streak. It was used in cave paintings when they wanted a red color. Hematite also can be polished and made into cabochons, but mostly, it is mined for its iron content and used for industrial purposes, such as for making steel or for X-ray shielding. Hematite is found all over the world, but primarily in Minas Gerais (Brazil), Cumbria (England), Morocco, Lake Superior, Utah (Thomas Range), and Arizona. This hematite specimen is from the UMKC Sutton Museum.

By the way, magnetic hematite jewelry is NOT made of hematite. The jewelry is a manmade ceramic barium-strontium ferrite magnet. See here: Hematite by itself is not magnetic, but some people think it is because it is frequently found with magnetite, which is magnetic.

Even Bigger Petrified Wood

A cylindrical red and orange piece of petrified wood that almost reaches the waist of the person standing next to it.

Photo by David Reed

Everyone had a great time visiting the Sutton Museum on our last field trip. Valerie just reminded me that I should be posting some photos from the museum so I will over the next week or two. When you come in the door of the museum, the first thing you will notice is this giant piece of petrified wood. A person is standing next to it for scale, so you can see it is about 2 feet tall. Much bigger than the other big petrified wood I wrote about last year.