Calcite rhombohedral cleavages: Iceland Spar
We sell natural semi transparent natural rhombohedral cleavages of calcite from Mexico as well as a higher quality variety of polished cleavages from Bolivia.
Natural Calcite cleavages from Mexico:
Our Mexican calcite cleavages range from fairly clear to slightly clear in parts. They are completely natural except that they have been broken from larger natural calcite crystals.
Polished Calcite cleavages from Bolivia:
These are of a substantially higher quality than our Mexican material. It is usually much more transparent and the cleavages have been polished on their surface. Even though they are more expensive, they have proved popular with our customers.
Calcite var. Iceland Spar.
Iceland spar is just calcite that is clear enough to show the property of double refraction. In practical terms this means that when you view a line or print through the calcite, its optical properties will make the line or print appear to be doubled. This is due to double refraction where the incoming light is broken into two separate components and reflected back out of the stone as two separate images. Double refraction is common in many minerals, but it is very pronounced in calcite and calcite is the most commonly available mineral that can be used to demonstrate this.
Calcite is one of the most abundant minerals found in the earths crust and there are thousands of localities where calcite can be found. A few of these localities produce Calcite crystals that are well formed and transparent. This variety of calcite has historically been called Iceland Spar. The reason for this is that in Iceland, in the 19th century giant crystals of calcite up to 7 meters were found. This deposit produced abundant material and gave this variety of calcite its name. This deposit has long been exhausted and no Calcite crystals have ever been found that were larger. A small percentage of these crystals were of transparent calcite. When these crystals were broken up to collect and ship them, they would commonly break into rhombohedral cleavage sections typical of calcite from any locality.
This ability of calcite and other minerals to break continually into smaller and smaller identically shaped cleavage pieces is one of the things that got scientists thinking about how small could you break them and that there must be some very small size beyond which you could not break/cleave them any smaller. This lead to the unit cell theory of mineralogy at an atomic level. Calcite also has the property of double refraction where light entering a crystal is split into two polarized light rays and this property is very pronounced in Calcite and is demonstrated by it, perhaps better than any other mineral. When you look through a piece of calcite at a line or a line of text on paper below it, the line or line of print appears toe be doubled.
These two properties, its cleavage and its double refraction make it eternally popular with earth science instructors to illustrate two common fundamental properties of many minerals. The more transparent the calcite the more expensive it is. Pieces of completely transparent calcite in large sizes (larger 200 grams) are in high demand for optical devices and can bring several more than 1000 dollars a kilogram in larger sizes. That is why we do not have any completely transparent calcite to sell.
Historically the best and most abundant source of this clear calcite was in Iceland and therefore gave it its name of Iceland spar. However there are very many localities for calcite that is clear enough to demonstrate this property and today probably the most abundant material comes from Mexico. Almost all the calcite that is used to demonstrate the doubly refractive properties of calcite are in the form of cleavage rhombohedrons, examples of which are pictured below. They are frequently only clear enough so that you can demonstrate the doubling property, but not completely transparent. The completely transparent and untwined variety that are in demand for optical instruments can cost more than $1000 per kg.
Our rhombohedrons of calcite/Iceland spar are ideal for demonstrating double refraction and for showing the classical rhombohedral shape. We carry two qualities of this material. The cleavage rhombohedrons from Mexico are of a lower grade and are less expensive, but we also have higher quality cleavage rhombs from Bolivia that has been polished in Brazil.