 PYRITE |
MINERAL STRIATIONS |
|
Striations are a product of a mineral's crystal habit. They can be very diagnostic and can even be an enhancement to the attractiveness of a specimen. They are incorrectly referred to as scratches, especially on transparent crystals.
The most common cause of striations are the convergence or juxtaposition of two crystal faces. One of the faces gets truncated or "overtaken" by the other, but manages to leave its mark in the form of an almost imperceptible edge or stria. This edge is repeated again and again as the mineral grows and can fill an entire face with these tiny edges or striations.
Generally striations are parallel, but some are triangular or even crossed. Lamellar twinning is also a common cause of striations. The twins boundaries make for a disturbance on the crystal edge. In lamellar twinning, the twins are repeated by the hundreds in a single crystal and these disturbances form the striations. Striations are generally common, but the best examples of striations are on the minerals quartz, pyrite, apophyllite, tourmaline, sphalerite and the feldspars, but there are many others.
Pyrite has striations on its cubic faces that are caused by faces that are not usually on the crystal at the time. Pyrite's striations are caused by a juxtaposition of two pyritohedron faces on the cube face. Generally the pyritohedron faces are not even formed on the typical cube habit, but their striations are there as straight lines. It is interesting to compare a perfectly formed pyritohedron with a perfectly formed cube of pyrite and see that the edges of the pyritohedron orient on the faces of the cube. It is interesting to notice that the striations on one side of the cube are perpendicular to the striations on the other side.
Apophyllite and tourmaline (such as schorl pictured above) have lengthwise or vertical striations and are caused by their respective juxtaposition of prism faces. Both of these lustrous minerals are greatly enhanced by the play of light across their striated prism faces.
Sphalerite is unusual in that it is isometric, but so complicated in crystal habit that only some of its faces are striated in different ways while others are perfectly flat in the same single crystal. The striations are the result of juxtaposition of several isometric forms. On its cubic faces, parallel striations are caused by the juxtaposition of two negative tetrahedron faces. On the negative and positive tetrahedral faces there are triangular striations caused by the juxtaposition of positive faces on the negative faces and visa-versa. Octahedral faces are generally free of striations in sphalerite. It makes for a rather complicated crystal.
The many different feldspars have lamellar twinning that causes their parallel striations. But their striations only show up on their cleavage faces and not on their crystal faces. Striations on the cleavage faces of feldspars is very diagnostic for this generally difficult to distinguish group of minerals.
 Amethyst Galleries' Mineral Gallery MINERALS |