Tuesday, December 20, 2016

Labradorite, a beautiful, dark-colored mineral with iridescence

Labradorite, which is derived from "red-hot" molten material called magma, is one of several phases (varieties) of the mineral plagioclase. All these phases have the same general formula (Ca, Na)(Al,Si)3O8. As the magma cools, a solid-solution series of different phases/varieties of plagioclase crystallize out sequentially, with varying amounts of Ca (calcium) and Na (sodium). The calcium and sodium ions mix in a continuous series with their ratio varying from 100% calcium and 0% of sodium, to the extreme opposite.

Labradorite consists of 50 to 70% calcium and occurs as blocky to lath-shaped crystals in calcium-rich igneous (magma-derived) rocks, such as basalt, gabbro, and anorthosites. Labradorite is relatively uncommon, but some rocks consist almost entirely of this mineral.
A 3 foot-high slab of labradorite used as a pedestal for a monument.
You can see how some of the crystals are large and lath-shaped.
 The bluish-iridescence of this mineral is especially evident  in the upper part of the picture.
One of the most memorable features of labradorite is its iridescent play of colors, which results from this mineral’s peculiar reflection of light. The reflection is caused by internal fractures that reflect light back and forth.

Polished piece (width 5.7 cm, 2.5 in.) of labradorite showing its iridescence. To see it, one must tilt the specimen at just the right angle to the prevailing light; a few degrees too much or too little tilt, and the iridescence disappears.
Labradorite-bearing rocks occur worldwide, especially in Labrador, Canada (where this mineral inherited its name) and in Norway.

Labradorite is used for making floor tiles, kitchen counter-tops, tables, and benches. It is also a popular gemstone.

For more information about the solid-solution series that is associated with the formation of plagioclase, please Google the term "Bowens Reaction Series." 

Note: I used to include links to topics covered in my posts, but recent changes in Google Posts now deactivate these links when posts go to the "Archive" file.

Saturday, December 10, 2016

Pyrite cubes

Pyrite is a mineral that most people have either heard about or seen. It superficially resembles gold, yet the chemical and physical properties of pyrite make it easy to distinguish it from gold. The main differences are listed below:

Pyrite is an iron sulfide, with the chemical formula FeS2. Gold's chemical formula is simply Au.

Pyrite crystal system is isometric (cubic), and crystals formed under perfect conditions will be cubes (as shown below). Gold is rarely found as crystals; rather, it occurs in nuggets, irregular blobs, or small flakes. It cannot occur in cubes.

Single cube of pyrite, width 2.9 cm (from Spain).
Cluster of intergrown pyrite cubes, total width 5 cm long (from Spain).
Pyrite is harder with a value of 6.5 on the Moh's Hardness Scale [i.e., a scale with talc and graphite the softest minerals (value of 1), and with diamond the hardest mineral (value of 10). Gold has a value of 2.5. Gold is very soft; so much so that other elements (e.g., copper, nickel, or platinum) have to be added to it (in the form of an alloy) in order to make jewelry.

Note: An ordinary steel knife (hardness value of 4.5) cannot scratch pyrite but can easily scar gold. The superior hardness and brittleness of pyrite also cause it to smash into bits if struck with the tip of a high-quality knife or shatter into small pieces, if hit with a hammer.

Pyrite's streak (its powdered from when scratched across an unglazed porcelain plate, called a streak plate), is black. Gold's streak is brassy yellow.

Small irregular piece of pyrite with its characteristic black streak on a "streak plate.
Pyrite is less dense, and small flakes normally wash away when placed under running water. Gold flakes are very dense and will sink. This is why "gold panning" works so well for finding gold.