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4MC2 - RUTILE paramorph after BROOKITE (twinned ) - $ 125 SOLD MAGNET COVE, ARKANSAS thumbnail, 1.8 X 1.5 X 1.5 cm
A sharp , cyclic- twinned brookite crystal replaced by lustrous rutile! Old classic material! Mke Howard had an article in Rocks and Minerals a few years back that talked about these crystals, in considerable detail. Basically, any "brookite" that is a pseudohexagonal bipyramid (not twinned) and that is bigger than a quarter-inch is virtually always replaced by rutile (this can be recognised by striations, that sometimes create a "moire effect", on the surface..You can also often spot a hint of the red internal reflections that characterize rutile, and the color is a little different from the black lustrous brookite xls.) I could be wrong and this one could be an exception - a very large brookite dipyramid. The striations I do see tell me it is a rutile.
4MC49 - RUTILE paramorph after BROOKITE - $ 150 SOLD MAGNET COVE, ARKANSAS thumbnail, 2 X 1.8 X 1.3 cm
A sharp , cyclic- twinned brookite crystal replaced by lustrous rutile! Old classic material! Much better in person. Mke Howard had an article in Rocks and Minerals a few years back that talked about these crystals, in considerable detail. Basically, any "brookite" that is a pseudohexagonal bipyramid (not twinned) and that is bigger than a quarter-inch is virtually always replaced by rutile (this can be recognised by striations, that sometimes create a "moire effect", on the surface..You can also often spot a hint of the red internal reflections that characterize rutile, and the color is a little different from the black lustrous brookite xls.) I could be wrong and this one could be an exception - a very large brookite dipyramid. The striations I do see tell me it is a rutile.
4MC54 - BROOKITE ON QUARTZ - $ 95 SOLD MAGNET COVE, ARKANSAS thumbnail, 2.3 X 1.4 X .8 cm | ||||||||
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Undoubtedly an old specimen, this piece is typical of a size and style said to be found here in the very early days, late 1800s to early 1900s. I have seen only a very few rutile specimens even approaching this size (in the Smithsonian and American Museums), and its degree of preservation and perfection is shocking (it is pristine and complete all around!). I have seen no brookite of this size, period, from here. This is a kind of pseudomorph replacement called a paramorph, now. It is no longer a brookite despite its apparent habit: A paramorph (also called allomorph) is a mineral changed on the molecular level. The mineral looks identical to the original unaltered form of the brookite (which is normally orthorhombic and not as heavy or robust as rutile) but is now replaced by another form of natural titanium oxide, (tetragonal) rutile . This is, I think, a MAJOR US locality specimen and a major rutile by any worldwide standard
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