Amethyst (#12)

A plate of lustrous gem to gemmy dark crystals, many phantomed, up to 5/8" in length.
North America Ten Mile District, Humboldt County, Nevada, USA 3 ½" x 2 ¼" x 1 ¾"

 

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Amethyst, as we have seen before (specimen #4) is the purple variety of quartz, SiO2. We have already seen the crystal structure of quartz (specimen #3) and discussed various colors of that mineral (#4, #9, and #10), so let's ask this question: Why is quartz so abundant in the earth's crust?

The answer lies, largely, in the abundance of the chemical elements in the crust of our planet. The 10 most abundant elements (symbol, % by weight) are: oxygen (O, 46.3), silicon (Si, 28.2), aluminum (Al, 8.1), iron (Fe, 5.4), calcium (Ca, 4.1), sodium (Na, 2.4), magnesium (Mg, 2.3), potassium (K, 2.1 ), titanium (Ti, 0.5), and hydrogen (H, 0.1 ). All of the other elements make up the final half percent. You may be surprised that there is so much oxygen in the rocks of the earth's crust, but of the 93 numbered specimens in this display, about 85% contain oxygen in the formulas of the minerals. It does not form a gas in the minerals, but is chemically bound to other elements.

Now, note that 0 and Si are the two most abundant elements. Quartz, SiO2, is made of those two elements, so maybe it isn't surprising that there is so much quartz in the crust of the earth. We classify minerals based on chemistry: silicates, carbonates, phosphates, sulfides, etc. The words that end in -ate contain oxygen in combination with another element, such as silicon, carbon, or phosphorus. Those that end in –ide don't, such as su lfides or halides (and oxides - more oxygen). Most of the minerals that make up the rocks of the earth's crust are silicates.

Notice that 82 of the 92 natural elements, including a large number of elements of importance to society, are in that last half percent. These have to be extracted from the earth - that is, mined. Now read what makes a mineral an "ore mineral" under specimen #10.