Page 10 - Demo

10|November 2019
Know Your Minerals
A Handy Guide to the Ingredients of Your Favorite Natural Stone
Slippery rock Gazette
Feldspar often forms rectangular or blocky-shaped crystals. In this slab of Ocre Itabira there are two types of feldspar. One is silvery white and the other is light brown.
Minerals are the com- ponents of all natural stones. The color of every natural stone, whether it’s jet black, glittery silver, or a kalei- doscope of Technicolor – comes from the individual minerals.
Minerals also give each stone its personality. Is it flashy, or subtle? Is it a uniform color, or a melee of diverse ingredients?
And, of course, minerals dictate the properties of a stone: Hard or soft; acid-resistant or acid-sensi- tive; flaky, chunky, or smooth.
Given that minerals determine so much about a stone, it warrants a look at some of the more com- mon minerals, how to spot them, and what they tell you about a stone.
Before we dive in, one important point is that you can only see indi- vidual minerals in coarse-grained stones. A smooth stone with small grains, like Absolute Black or Pietra Grey, doesn’t reveal much about specific minerals because you can’t see them. But many popular stones have big crystals in all kinds of patterns and colors, inviting curiosity about just what those minerals are all about.
Feldspar
Feldspar is the most abundant
Karin Kirk
usenaturalstone.com
Many thanks to Slabworks of Montana and Montana Tile and Stone for allow- ing me to explore and photograph their beautiful stone slabs.
mineral in the Earth’s crust, but it’s far from mundane be- cause it occurs in a huge range of colors and forms. When you look at a slab of typical granite, you’re looking at mostly feldspar. Igneous rocks like granite take shape as they solidify from liquid magma. You can think of magma as a ‘slushy’ drink. It’s a mixture of solid bits and liquid. Feldspar crystals are often the solid chunks within a body of slushy magma, and you can see the chunky tex- ture of feldspar in some granite slabs.
In other cases, feldspar is al- tered by metamorphism, changing its shape from pushing, pulling, or shearing. In these cases, the blocky shapes of feldspar crystals can become more rounded.
Color:
Feldspar crystals can be white, black, and any shade of grey. They can also be pink, cream, brown, and sometimes green. Best yet, the variety of feldspar
called labradorite is iridescent blue, and plays the starring role in Blue Pearl and Volga Blue.
Identifying Features:
Since feldspar can be nearly any color, using color to identify it won’t help at all. But it does have a few features that make it recognizable.
• Feldspar is not glossy and its lus- ter is similar to porcelain when its not polished.
• It is always opaque, meaning, you can’t see ‘into’ the crystal at all.
• It’s more or less rectangular in shape.
• The crystals break into naturally flat faces called cleavage planes. This is especially visible on a honed or leathered slab when you look at it from an angle. The flat faces will catch the light. This is also evident on the edge of a slab where you can see a crystal in three dimensions.
• You can sometimes see subtle stripes or grooves in a feldspar crystal.
Blue feldspar — called Labradorite — in a slab of Volga Blue.
These feldspar grains lost their blocky shape when the stone got squeezed and stretched during metamorphism. The black, wavy layers are black mica.
In this slab of Sea Foam Green it’s plain to see that the feldspar crystals floated around in a liquid magma.
Properties: Feldspar is an all- around easygoing mineral. It has no special needs, has a Mohs hardness of 6 to 6.5 (harder than glass), and holds up well to hard use. It won’t etch, flake, or offend anyone in any way.
Examples: Alpine White, Antique Brown, Patagonia, Coral Grey, Blue Pearl, and many others. Since feldspar is such a
common mineral, there are doz- ens of examples.
Superpower: Feldspar makes you more gracious and less self-involved. The relaxed flow of energy from feldspar allows you to let someone else take that last slice of pizza, even though you’re still a little bit hungry.
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