Common Mineral Terms
Pseudomorph (replacements): To change from one to another. For example, pyrite can pseudomorph from pyrite to goethite. Goethite is chemically pyrite but with a different crystal structure. The change is caused by ground water "rusting" the pyrite. Azurite replacing malachite, and orpiment replacing realgar are other great examples.
Skeletal or Casts: This is when a mineral coats a different mineral and the one that is coated dissolves away leaving its shape under the coating mineral. This happens more often than one might think. I have seen prehnite form over glauberite and then the glauberite dissolves away. Calcite over calcite is another common one. I have seen some great native copper specimens that formed over quartz and barite crystals. Skeletal usually refers to specimens that are still hollow from the first mineral being gone.
Phantoms: This is when you can see the external crystal shape as a shadow on the inside of a crystal. For example, you can have a quartz crystal and when you view it you can see what looks like quartz crystal shapes on the inside. This is caused by the growth of the crystal. The conditions that cause a crystal to grow may be interrupted. While it is not growing, a layer of another mineral grows over the top of it. Then the main crystal starts to grow again leaving an outline of the initial growth. In quartz, chlorite is the most common phantoming mineral. This is very common, especially in quartz and fluorite.
Zoning: This is when you can see a single crystal change colors showing what looks like a phantom. Tourmaline is really good at this. You can have a single crystal of tourmaline and when you slice it up you can get a watermelon effect where the outside is green and the inside is pink (hence "watermelon"). Some tourmalines can be sliced and change with each slice. Madagascar produced some tourmalines that were 3+ feet long and when sliced in ½" slices they produced a stunning effect of 6 different color zoning in the center--black on the outside then triangles inside each other set 180 degrees to each other with blue, white, green, yellow, and lavender as you get closer to the inside.
Enhydro: Many, maybe even most, crystals have small water pockets on the inside. When they are large enough to see and have an air bubble that moves they are termed en-hydro. Quartz and gypsum are the most common examples. Rainbow obsidian gets it's color this way. It has thousands of very small water filled pockets that are stretched from the flowing of the lava. Once cooled. the pockets of water act as a prism creating the rainbow effect.
Twining: Crystals can form in interesting combinations. As discussed in a previous newsletter, calcite can form thousands of crystal habits. Twining is when you have 2 different crystals that form to make one crystal. Cerussite may be the most famous. It can form a sixling that looks like the inside of a bicycle wheel (six spokes sticking out from the center). Staurolite forms perfect crosses.
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