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At first, many people find luster a bit confusing. Luster is the way that an object reflects light, and although the different types of luster can be difficult to describe, your daily experience makes them easy to recognize. Once you understand luster, you will recognize the common lusters instantly.
I am sure that you have seen breakfast cereals that claimed to be enriched with iron. You may have thought that the manufacturer added some extract from spinach or liver, but the actual ingredient may surprise you.
This week's experiment came to me from my good friend Bob Cox. He told me about the trick and wanted to know why it worked. It took some thought and testing to come up with a theory of what is happening and then several e-mails to experts to confirm that I was on the right track.
When we tested hardness, we were trying to scratch the specimen, not break it. For the next two tests, fracture and cleavage, we will be breaking the specimens. We are not looking at how difficult it is to break them. Instead, we will be looking at how they break, especially at the shape of those broken surfaces. Minerals that break in flat, smooth planes have a property called cleavage, and we will look at those in the next test. Any kind of break that is not cleavage is called fracture, and there are several different kinds.
One of the most important tests for identifying a mineral is determining its hardness. After we determine the luster of a mineral, hardness is the test we use to narrow down the possible choices.
These are ripple marks in beach sand, made by the wind. Similar ripples can be seen in deserts, and even in large playground sandboxes. What causes the ripples to be so evenly spaced?
