Stone of the Week - Sodalite

Sodalite derives its name from the sodium content of this mineral; the chemistry of this mineral is Na4Al3(SiO4)3Cl, or Sodium Aluminum Silicate Chloride. It belongs to a group of minerals called feldspathoids. Feldspathoid minerals have poor silica content, and in the case of sodalite, it has absolutely no silica (aka quartz) at all. In fact, this mineral doesn’t even form if there is quartz around – it would react with the quartz to form another mineral. Since quartz is so abundant on Earth, there is sodalite is not a common mineral.

Sodalite often appears as a mixture of colors, and rarely found in a solid blue mass.


Many are familiar with the royal blue color of sodalite, however it occurs in a variety of colors including shades of blue, color less, white, light yellow, green, gray, pink, and violet. Hence it can sometimes be difficult to identify sodalite from other minerals. In its commonly sold blue form, one way to distinguish it is its streak. This the color the powder of the mineral leaves when ran across a ‘streak plate’ which is commonly a piece of white porcelain, however in a pinch you can try doing this on a white piece of paper. *Be sure to do this using a ‘fresh’ surface of the stone – trying this with polished surfaces such as what you get with beads doesn’t work – you need to expose a fresh surface of the stone.


The blue/blue+white variety is most often used for creating beads and decoration.


Sodalite is only found in quantity in a few select locations – mainly in Canada, and a few states in the USA, though a few smaller deposits can be found on other continents. There are a few different types of sodalite, one of the more interesting being Hackmanite. This particular form of sodalite has a property called “tenebrescence” which results in the stone changing color when exposed to light! (This same property is used in industry to create those glasses that will darken when exposed to sunlight!)


This mineral can make for some very pretty and unique jewelry!


Sodalite is a beautiful stone today used mainly for jewelry work, and sometimes decoration and sculpture. One story has it that in 1901, the Prince and Princess of Wales (the future King George V and Queen Mary) were presented with a piece of sodalite. The princess loved the beauty of this stone so much that she had tons of it shipped back to England where she used it to decorate the Marlborough house. Years later this mine was renamed the “Princess Sodalite Mine” in her honor.


Sodalite Facts:
Chemical composition: Na4Al3(SiO4)3Cl
Crystal System: Isometric
Color: blue, color less, white, light yellow, green, gray, pink, and violet
Habit: Massive, rarely crystals
Fracture: Uneven, Conchoidal
Cleavage: Dodecahedral (six directions), poor
Luster: Vitreous, Greasy
Transparency: Crystals are transparent to translucent, massive specimens are opaque
Hardness: 5.5-6
Specific Gravity: 2.27-2.33
Streak: White
Occurrence: Worldwide, but most commonly in Canada and the US
Other: Fluorescence



References

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia - Sodalite. http://en.wikipedia.org/wiki/Sodalite

Wikipedia - Tenebrescence. http://en.wikipedia.org/wiki/Tenebrescence

Mineral Galleries - Sodalite. http://mineral.galleries.com/minerals/silicate/sodalite/sodalite.htm

Geological Garden gets 4-ton rock. http://newsrelease.uwaterloo.ca/news.php?id=1004

Adventures in Geology - Cedar Sink, KY

So my first field trip this semester was last week to a place called Cedar Sink, Kentucky. This was part of a lab out in the field for my geomorphology class. Geomorphology in general is the study of landforms - including classifying them, what processes form them, and sometimes the interactions between them and humans (how geomorphology can affect us, as well as how we affect the landscape).

Cedar Sink from an overview point.

Cedar Sink is located in south central Kentucky, inside the Mammoth Cave National Park, which is also the location of the largest known cave in the world, Mammoth Cave. Cedar Sink is aptly named because it is a collapse structure – also called sink holes or sinks. That is because this region is karst terrain, where the landscape is influenced by the dissolution of rock (essentially dissolves). When the rock is dissolved it can result in the formation of caves, as well as a variety of other features including sinkholes which are depressions in the ground formed by the removal of soil or rock.

A view of some of cave areas and one of the sinkholes filled with water. Blue arrow is pointing toward my professor to give an idea of the scale.

However, we weren’t there to study the sinkholes, but rather the hill slopes. The main point of our lab was to do sketches or profiles of the hill slopes. We also tried to determine what processes created them, as well as any that are resulting in the removal of the material on the hill slopes. These kinds of processes are important to understand, as they affect other processes such as groundwater movement, and can have an impact on human activity as in the case of land slides and rock falls.

Another side of the area that leads into a cave. As you can see from the ice cicles it was very cold that day!