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Click the thumbnails for larger pictures.
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Stilpnomelane veins and crystals (black) in
massive quartz and green chert at Blanchard Mountain, Skagit County,
Washington. |
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Colorful Goethite on Pyrite crystal from
Hanson Creek area. |
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Hyalite (or is it
Cristobalite?) on basalt from Table Mountain, north of Ellensburg.
It fluoresces bright green under short-wave UV, indicating traces of the
uranyl ion. Hyalite (properly known as
Opal-AN) is a clear
form of Opal.
Cristobalite is a form of
silicon dioxide with a different crystal structure than quartz. Two different
expert geologists have told me that this is 1) cristobalite, 2) hyalite. What's
the difference?
| Property |
Hyalite |
Critsobalite |
Sample |
| Water? |
Yes |
No |
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| Crystal Structure |
Primarily amorphous,
may have a network-like structure |
Tetragonal |
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| Hardness |
5.5 - 6.5 |
6.0 - 7.0 |
6 |
| Fluorescence |
Usually - bright
green from uranyl ion |
Occasionally |
Bright green |
| Habit |
Transparent and
globular |
Usually translucent
and white, occasionally transparent and globular. |
Transparent and globular |
It certainly looks like photos I've seen of
hyalite. On the other hand,
perhaps both are technically correct since opal is not a true mineral, but is
composed of cristobalite and amorphous silica.
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A quartz crystal Fiona and I found on a
Geology Adventures "Crystal Kid" trip. |
Our Blue Mineral Collection
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