At the end of January, I drove from Florida to Arizona to purchase minerals for the Gitche Gumee Museum’s gift shop. I used to go to the Tucson Gem and Mineral Show every year. Now, to minimize expenses, I go every three to four years. But it is always to go to this huge show (which is actually more than 40 separate shows located all over the city)!
One of the minerals I purchased this year for the first time is a combination rock from the Democratic-Republican of the Congo (formerly Zaire). This country of more than 100 million people is in the middle of the African continent.
In the past, the museum has sold Chrysocolla and Malachite but never had I had the privilege of selling this combination rock. The green in these specimens is malachite; the blue is chrysocolla. There is also black heterogenite included in some of the specimens. These minerals are secondary-fill copper minerals that form deep underground.
Chrysocolla has a blue-green/cyan color and is a minor secondary ore of copper that formed when copper ores were altered by mineral-rich fluids
Malachite also gets its color from copper, which makes up about 58 percent of its content.
Technically, malachite is a “secondary mineral,” which means it formed when copper-rich rocks were dissolved or chemically altered by circulating fluids. Malachite’s characteristic swirling and concentric patterns are a result of this formation process. This beautiful mineral was mined in Egypt over 6,000 years ago. Not only was it used as both a gemstone and a decorative stone, but it was ground into green pigments for painting and cosmetics. Synthetic green pigments equaling the vivid color of Malachite were not developed for thousands of years until the Industrial Revolution. But if you decide to work on polishing and cutting malachite, BE CAREFUL! The copper content of the dust released from grinding malachite is toxic! Please keep the rough stone wet and wear protective respiratory gear.
Chrysocolla-Malachite is a combination of two copper-derived silicate minerals. Since these specimens contain different minerals, they are considered rocks. Minerals are consistent in their make-up; rocks contain different minerals and are not consistent in how much of each mineral they contain. An easy way to understand the difference is that rocks are like cookies: they contain different ingredients such as flour or sugar (which are like minerals). The sugar making up cookies may be the same, but other ingredients in cookies change – so cookies are not always the same!
In the case of Chrysocolla-Malachite, it is a combination of two Copper minerals. Copper typically formed as a sulfide, which oxidized when exposed to water. If that water also contained carbonic acid, it dissolved the copper and formed new compounds, including green malachite. If the copper was oxidized, cyan-colored chrysocolla also may have formed. Sometimes these two situations happened in the same spot and at the same time resulting in both chrysocolla and malachite developing next to each other and intertwining – which is what happened to form these combination rocks in the Democratic Republic of the Congo.