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Quartz - Dauphine twin
Based on a specimen from Uri, Switzerland
The Dauphine twin law is a parallel axis twin combining either two left or two right handed quartz crystals. Quartz is commonly twinned according to the Dauphine twin law, but specimens exhibiting ideal form are less common than might be expected. Faces that would reveal twinning are not always expressed, and twin boundaries are often irregular and hard to see. That means in a crystal whose faces don’t reveal its handedness, twinned regions with seamless boundaries can pass unnoticed. (Models of quartz crystals whose handedness can be determined are shown here)
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Quartz - Brazil twin
Based on a specimen from Canton of Graubunden, Switzerland
The Brazil twin law is a parallel axis twin found in quartz, combining a right and a left handed individual. Ideal drawings of a fully developed, symmetrically twinned crystal are often found in textbooks, and make for some really lovely diagrams (see below for one of them). Unfortunately, actual crystals with these faces are extremely rare, but it’s possible that I have one in my collection. More about that below, but first here’s a wood model representing the ideal form, depicted in fig. 455 from Dana’s A Textbook of Mineralogy, 4th ed., edited by W. E. Ford.
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Quartz - Right and Left Handed
Based on a specimen from Uri, Switzerland
I have spent a lot of time over the years digging through bins of cheap quartz crystals looking for right and left handed examples. After a lot of searching, I have a number of each, but most of those crystals are only singly terminated, damaged, or without a location. When I started making models, I decided I was in the market for a better specimen, and happened across this beautiful left handed doubly terminated crystal with textbook form at a show a few years ago. The models shown below are based on this specimen.
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Calcite - Scalenohedron with Rhombohedron Faces
Based on a specimen from Joplin, MO
The calcite crystal this model is based on has an attractive combination of forms, with the added bonus of a sharp orange phantom inside. On the surface, the faces of the rhombohedron {02.1} bevel the edges of the scalenohedron {21.1}, while the phantom appears to be the scalenohedron alone. This model shows only the surface forms, although I am contemplating the future construction of a glass or plexiglass model with a visible phantom model suspended inside.