Classifying Minerals

CLASSIFYING MINERALS

While you can classify rocks as pretty or interesting, you can't use those terms when sharing legitimate information about the rock or mineral before you. Here are some good classification categories that will help you both identify and talk about rocks. Most minerals can be identified by their visible properties, although real mineralogists often use microscopy to nail down the details of the mineral.

When classifying minerals, look for the color you see, the streak, and the luster first. Color actually doesn't help much because the same mineral can be different colors. In addition, you can have real gold looking a lot like fool's gold, which is iron pyrite. Quartz is purple if it has a bit of iron in it; if not, it will be clear in color. Purple in quartz means it is really called amethyst.

Next, check the streak, which is the mineral's powder color. It doesn't vary nearly as much as color so it is a better measure of what the mineral really is. Quartz has no streak; many others don't either. Simply scrape it across any unglazed white porcelain plate. Pyrite, which is gold in color, has a black streak, while real gold has a goldencolored streak. Hematite is black but has a reddish brown streak.

Luster is how well the light shines off a mineral. You can also describe it as how metallic it is or non-metallic. Iron-pyrite has a metallic luster as the light shines off it. Quartz is non-metallic. Here are some common lusters and what they really look like in layman's terms:

• Amantadine – sparkly appearance like a diamond

• Earthy – dull or clay-like appearance

• Resinous – looking like tree sap resins such as sulfur

• Pearly – looking like a pearl

• Silky – having long soft fiber-like appearance

• Vitreous – looking like glass or like quartz

• Metallic – looking like a shined or polished metal

Hardness is an interesting feature of a mineral. Aren't all rocks hard? It turns out that you can measure the hardness in the field if you need to. You just need a few reference samples for comparison. The Mohs hardness scale is a great way to do this. We will talk about how you can do it too in a minute.

Next, we get cleavage and fracturing of a mineral. Crystals are easy to break in certain shapes and directions but not in others. How a mineral fractures will be unique to that type of mineral. Minerals with cleavage will break along certain planes but not others. The planes will be a smooth surface. You know, for example, that salt crystals are cubic if you've seen them up close. This is what halite looks like as well when you cleave the rock. Mica cleaves into sheets but just in one direction. If you try to cleave it the wrong way, you simply get mush.

Other minerals are more interesting and form octahedrons or other interesting shapes. Many gemstones are cut along their cleavage lines to achieve the most attractive appearance. Diamonds like to cleave as octahedrons, for example.

A fracture is not the same as a cleavage plane. Fractures break the mineral in ways not along the cleavage plane. Minerals are fractured in different ways unrelated to the substance in the mineral. Metals will fracture with splinters or with jagged edges, depending on the situation. Quartz will fracture with smooth or curved surfaces.

There are so many different characteristics you can have with regard to any given mineral that might identify it. Here are a few:

• Can it fluoresce under UV light? It might be fluorite.

• Is it magnetic? It might be magnetite.

• Is it radioactive? Don't panic. It is probably uraninite.

• Does it bubble in acid? It's probably calcite.

• Does it have a rotten egg smell? It is likely sulfur-containing.

• Try tasting it – halite is salty.