This excerpt comes from The Quartz Page and is offered here with permission. There are beautiful pictures on the website (that are not permitted to share commercially), and it is a very informative resource.
Skeleton QuartzIn skeleton quartz the edges grew more quickly than the faces, so the edges stand out like the frames of a window. Crystals that grow very quickly often develop skeletal growth forms -- other examples are gold and rock salt crystals with hopper growth or snow flakes.
In German literature this growth form of quartz is known as "Fensterquarz", and the translation, window quartz is gaining popularity, as it is - at least for quartz - a more descriptive term. Occasionally the terms frame quartz and cavernous quartz are used. The term skeleton quartz is indeed a bit confusing as the crystals have not been "skeletized" by dissolution.
The faces on a skeleton quartz will grow from the edges to the center. Sometimes these faces simply grow as thin transparent plates, then usually several generations of them can be found, separated by empty spaces. Should these plates finally be completed, the watery solution inside will be trapped behind a "window".
Skeleton quartz often shows unusual surface patterns and curved distortions of the faces. These might look like fractures but are growth forms.
Skeletal growth is indicative of an unrest geological environment with rapidly falling temperatures and/or pressures. Many skeleton quartzes show inclusions of clay and other detrital pocket material, often captured in large cavities. The sedimentation of fine pocket material takes place quickly compared to the speed of growth of crystals, even that of skeleton quartz, so one would not expect any detrital inclusions in the outer layers of a crystal. The inclusions of clay might be explained by tectonic activity shaking up the fine detritus, or by a boiling watery solution in which this detritus cannot sediment. The fluid and gas inclusions that are often found in skeleton quartz seem to support the latter view, but many biphasic inclusions (that consist of a gaseous and a liquid phase) were monophasic (either liquid or gaseous only) when they were enclosed and became biphasic inside the crystal at lower temperatures.
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