Page 18 - Demo

18|January 2020
Know Your Minerals - Part Three
A Handy Guide to the Ingredients of Your Favorite Natural Stone
Slippery rock Gazette
INour last installment of Know Your Minerals, we will look at some of the less glam- orous minerals that can be seen in the slabyard. As you may recall, you can only see individual min-
erals in coarse-grained stones.
Amphibole
Black flecks in an otherwise light granite are either amphi- bole or black mica. Amphibole and black mica are also the duo that makes up the dark stripes in gneiss. (The light stripes are feld- spar and quartz.) Amphibole is also called hornblende.
Identifying features:
•Amphibole is an even, jet black.
•It tends to form crystals that are skinny rectangles, but sometimes it also has an irregular, blocky-ish shape.
•You can tell the difference between mica and amphibole by shape. Mica = thin flakes, and amphibole = thin rectangles. It’s subtle, but different enough to be a reliable way to tell these miner- als apart.
•When in doubt, look at the edge of the slab to determine the crystal shape.
Properties:
Amphibole has a hardness
between 5 and 6, which is similar to or slightly harder than glass. It takes a polish well, and does not need any special care. In most stones it’s a minor ingredient.
Examples: Amphibole is usually a minor ingredient in slabs, but it makes a spectacular appearance in Crowsfoot Schist, Montana Brown, and Alaska White. Stones like Barcelona, Roca Montana, and Bianco Antico have more subtle crystals of amphibole.
Superpower: Eases headaches; but also may cause cravings for salty things or Chinese food.
Calcite
This humble mineral expresses itself in so many wonderful ways. Calcite is the main ingredient in marble, limestone, travertine, and onyx.
Color: Calcite is almost always white or nearly white, and it can have tones of other colors like
Karin Kirk
usenaturalstone.com
Many thanks to Slabworks
of Montana and Montana Tile
and Stone for allowing me to explore and photograph their beautiful
stone slabs.
cream, apricot, light brown, light green, light grey, or light pink.
Identifying features:
• Calcite can look a bit like quartz (hence the perpetual confu- sion between marble and quartz- ite), but it has a few differences.
• Calcite has a satin luster, while quartz looks glassy and is more translucent.
• Calcite forms crystals with flat surfaces, and also breaks along flat planes. So when light reflects off calcite, you see glints of light from flat surfaces (see photo).
• When in doubt, go by the properties rather than the looks.
• Calcite is easily dissolved in groundwater and it often fills in cracks in rocks. Bright white stripes in dark colored limestone or marble are veins of calcite.
•Just to make things more con- fusing, quartz can also make light colored veins in a stone. A quick hardness test with a pocketknife will reliably tell you which is which. Please turn to page 30
Amphibole showing off its characteristic black rectangles. The white is feldspar, and the copper color is quartz. The small flecks of black just above the left side of the pencil are black mica. This stone is called Sedna.
Left: Amphibole crystals are the dark brown or black rectangles. The clear, grey, glassy vein is quartz. There are also white feldspar crys- tals throughout this slab, and some garnets near the bottom of the photo. This is Montana Brown.
Above: Don’t ask me to explain how this piece of amphibole got this inter- esting shape. But it sure is cool! The slab is Alaska White.