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| Looking across Clear Creek Valley to South Table Mesa from one outcrop of amygdalar basalt. |
Zeolites are microporous minerals formed from aluminum and silica. Microporous means that their mineral structure has large holes in it, and this feature gives way to the usefulness of zeolites. They can be used to purify water by the means of ion exchange beds that consist of zeolite minerals. For the same reasons they are commonly used in laundry detergents.
Although the zeolites found on Table Mesa aren't useful for purifying water, they are quite spectacular in their occurrence. The delicate and intricate zeolites from this locality are prized rare specimens that rarely occur together in other parts of the world.
They are found in the amygdalar basalts in this locality. Amygdalar simply means that there were holes (or vesicules) in the rock that have been filled in by some material, which is usually calcite or zeolites. If there were no material in the holes, it would be simply a vesicular basalt. The zeolites are reported occur in seven different varieties, but the most common seem to be thompsonite, analcite, chabazite, okenite and mesolite.
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| A cavity of thomsonite (the tan balls) that has been opened and exposed to weathering. |
The basalts of Table Mesa are very difficult to break up. Attempting to break these rocks with a 16 or 22 oz. rock hammer would be rather futile, and its doubtful that your hammer would weigh the same after so much steel would get taken off. Even my chisels would show a new imprint of the sledge's face after each impact, and the top of the chisel would mushroom out a bit and curl back with each hit. Usually, after smashing at a face for a few minutes with the sledge and chisel, you will get a fracture in the rock.
After a fracture has been propagated, which usually tends to be along the long axis of the zeolite pockets, you have to go and break out the ends of the fractures so that the rocks can be removed. Otherwise, they all fit in just like puzzle pieces that despite being loose do not want to be removed. After demolishing the rock to removable pieces, you get the pure pleasure of removing each piece and inspecting it for crystal-filled pockets.
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| This amygdule contains analcite, chabazite and thomsonite. |
Sometimes you get nothing, and sometimes there are some large cavities. One indicator of finding pockets seems to be oxidation of the rock immediately surrounding the pocket. These little trails of oxidation can lead your hammer from one pocket to the next.
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| I believe this to be an okenite crystal (~2.5 cm) among chabazite (square white and clear crystals), this specimen (in-situ) was destroyed in the attempt to retrieve. |
The zeolites are somewhat zoned, and in some areas different types of zeolites will be found more frequently. For example one area was characterized by large cavities of purely thomsonite. Other areas had cavities filled with thomsonite and analcite, and some contained purely chabazite. The amygdules in the thompsonitic area were commonly 6" long and 2-3" in height and tending to be stretched in a NW/SE direction. The largest was over a foot long and about six inches tall, with two separate cavities attached. It had however been broken out a long time ago and was therefore quite weathered.
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| Chabazite crystal showing its pseudo-cubic structure. |
Also of note was the immediate change of color when the minerals are brought out of pocket. Within five minutes of exposure to air, my translucent green thomsonites had dulled to a tan coating outside. The more weathered thomsonites are completely tan. Also the chabazites, which in their cubic form resemble NaCl or salt crystals, are clear and transparent when first broken out but quickly gain a slightly white coating on the crystal after exposure.
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| An analcite crystal with thomsonite on basalt matrix, the analcime is quite large for this locality(~1.5 cm wide) |
Although a lot of effort oft results in cracked and damaged crystals, there is a technique to collecting these crystals from the pockets. A hard day's work may result in only one or two nice specimens, and sometimes the best ones escape you. If you decide to head up to Table Mesa, be sure to check that collecting is allowed before you go. Bring along some soft packaging to transport your crystals back to the car, because the jolting of carrying them can also damage the crystals.
