Fabulous feldspars!

Aluminum tectosilicate: A big name for a big group

God created a very big world, and it is full of diversity. From the discovery of new plants and animals to the identification of new minerals, there are plenty of hidden things still out there to find!

One of the most diverse groups of minerals is the feldspars. You have seen them when looking at the natural stone countertops at the local home improvement store. They often form the majority of the rock, providing the pink, white, gray, or bluish hues that give us the overall look and feel of the stone.

While they display a range of colors and can be found in many different kinds of rocks, feldspars have a very consistent set of physical properties. For example, they are all fairly hard and break cleanly in two directions. This is ultimately due to their chemistry.

Below we'll learn how the major feldspar groups are categorized, then explore three important feldspar minerals. And the next time you see a stone countertop, look closely. I'd expect you'll find one of these three minerals in it. Let's explore!


Three elements, two groups

All feldspars are made with a basic unit of aluminum, silicon, and oxygen. This unit gives the feldspars their durability. To make so many different feldspar minerals, these units link up with other metal atoms like potassium, sodium, and calcium. The three-part diagram to the left shows the minerals that form out of these chemical options. The minerals form two groups: the alkali feldspars (left) and the plagioclase feldspars (bottom).

Starting from the top

The alkali feldspars add sodium and/or potassium to the aluminum-silicon-oxygen unit. To the right are two specimens of the potassium-rich feldspar called microcline, which appears at the top of the chart above. Microcline is often a pale orange or pinkish color, but can also be gray or even blue-green. You'll see pink/orange microcline it in many granites.

White as snow

The image at the top of this email is the feldspar called albite, which is also seen in this granite to the left. The name albite comes from the Latin word "albus", which means "white". Pure albite is very rare; it often forms together with its neighbor clevelandite. Both are part of the plagioclase feldspar group, though albite is also the sodium-rich member of the alkali feldspars.

Shimmering beauty

Among the countertops at the home improvement store, you'll certainly find a rock containing the plagioclase feldspar labradorite. Ranging from green to purple to a dark smokey gray, labradorite has about a 50-50 mix of sodium and calcium with its base unit. As a result of variations in the crystal structure, these minerals can show off eye-catching iridescent flashes of color (green, blue, and purple). Geologists call this "labradoresence."

And that's just the beginning!

The diversity of feldspar minerals is even wider than I've let on. The alkali and plagioclase feldspars we've explored are rather common, but there are rarer types made with other kinds of metals, like barium, boron, and lithium. Some of these are important to our current technologies (like lithium batteries), or their chemistry is perfect for making certain types of glass (soda glass and borosilicate glass, for example). Perhaps we'll have to look at these practical minerals again to learn some more...


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Many Blessings,

Marcus R. Ross, Ph.D.

 Science, Faith, and FUN!!!

  • Jul 26, 2021
  • Category: Geo-Bytes
  • Comments: 0
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