POPULAR PSEUDOMORPHS - More Common Varieties Enhance Any Collection
Rock&Gem Magazine|January 2021
In part one of this two-part series, which appeared in the December 2020 issue of Rock & Gem, we explained pseudomorphs as minerals whose normal form has changed, so they may look like the original but are now another mineral.

That change results in pseudomorphs, sometimes called pseudos, including replacements, casts, paramorphs and others. Some of these pseudomorphs are very attractive and common enough to add to any collection.

This article will describe some of the common and attractive pseudomorphs that are most likely available to collect. These examples include minerals and lapidary materials like petrified wood, as described in the first part. The most common lapidary material that can contain pseudomorphs are agates, especially banded agates. We may not think of agates as having pseudos, but many do.

Banded agates form in gas vesicles or openings in volcanic rock. One theory says silica-rich waters invade these open pockets and deposit the silica as alternating bands of color. Another theory suggests silica gel in the still fluid volcanic rock accumulates slowly to form rounded masses that cool, forming agate banding. In either case, the solutions in the cavity also contain molecules of other minerals like carbonates and zeolites, which tend to crystallize first on the walls of the hardening volcanic rock. The formation of the agate bands follows, setting the stage for pseudomorphs to develop.


As the still hot silica-rich solution slowly cools, it may replace the already formed carbonate or zeolite crystals. The result is silica ps carbonate or zeolite, which is visible when the agate is cut. Some of these pseudos are hexagonal, most likely a carbonate mineral calcite or aragonite. Several zeolite minerals form as radiating spherules of needle crystals or longer single crystals and can be replaced by silica and develop small radiating shapes attached to the vesicle wall. Silica can change them into pseudos. Another interesting feature in some agates is holes left when long slender needle crystals form, are engulfed by the agate then dissolved away, leaving a tube-like opening or cast.

One of the most common and abundant sources of colorful pseudos is malachite ps azurite found in many desert copper mines. In these deposits, weathering can penetrate to depths of 1,000 feet or more forming secondary minerals. These impure waters, often acidic, attack primary copper minerals like chalcopyrite and the host rock, mostly limestone. Often, a host of copper develops, including azurite, malachite, cuprite, native copper, and chrysocolla. As part of this process, pseudomorphs become inevitable and form copper after cuprite, chrysocolla after azurite, or malachite after azurite, the most common colorful copper carbonate pseudomorph.

The molecular structure of azurite involves three positively charged copper cations, two negatively charged carbonate radicals, and two negative OH radicals. Malachite’s molecular structure is simpler, two copper cations, one carbonate radical, and two OH radicals. Of these two copper carbonates, azurite is slightly less stable. When weathered, it can give a copper atom and a carbonate radical, resulting in it becoming malachite. This change in the chemistry also alters the color, from blue azurite to green malachite, while retaining the original monoclinic crystal form of azurite — which is the most common and most abundant copper mineral collected.


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