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Calcite exhibits several twinning types that add to the observed habits. It may occur as fibrous, granular, lamellar, or compact. A fibrous, efflorescent habit is known as lublinite. [11] Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain.
Halite (or salt) has cubic cleavage, and therefore, when halite crystals are broken, they will form more cubes. Rhombohedral cleavage occurs when there are three cleavage planes intersecting at angles that are not 90 degrees. Calcite has rhombohedral cleavage. Octahedral cleavage occurs when there are four cleavage planes in a crystal.
On the more perfect cleavage, which is parallel to the basal plane (P), is a system of fine striations, parallel to the second cleavage (M). Crystal twinning occurs when two or more adjacent crystals of the same mineral are oriented so that they share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth ...
Since the nature of cleavage is dependent on scale, slaty cleavage is defined as having 0.01 mm or less of space occurring between layers. [1] Slaty cleavage often occurs after diagenesis and is the first cleavage feature to form after deformation begins. The tectonic strain must be enough to allow a new strong foliation to form, i.e. slaty ...
Minerals with many cleavages might not break equally well in all of the directions; for example, calcite has good cleavage in three directions, but gypsum has perfect cleavage in one direction, and poor cleavage in two other directions. Angles between cleavage planes vary between minerals. For example, as the amphiboles are double-chain ...
This is clearly seen, for instance, in the Wollaston prism which separates incoming light into two linear polarizations using prisms composed of a birefringent material such as calcite. The different angles of refraction for the two polarization components are shown in the figure at the top of this page, with the optic axis along the surface ...
These angles are characteristic of the system to which the mineral belongs, and often of the mineral species itself (see Crystallography). To facilitate measurement of extinction angles, various types of eyepieces have been devised, some having a stereoscopic calcite plate, others with two or four plates of quartz cemented together.
To find this, simply line up the cleavage lines/long direction with one of the crosshairs in the microscope, and turn the mineral until the extinction occurs. The number of degrees the stage was rotated is the extinction angle, between 0-89 degrees. 90 degrees would be considered zero degrees, and is known as parallel extinction.
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