Monday, July 10, 2006

Mineral Specimen Terms You Won't Want to Be Without

Some common and not so common terms that are often used with minerals and
rocks include:

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. 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 zonings 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 enhydro. 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 news letter, 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

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