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The Mohs hardness of all plagioclase species is 6 to 6.5, [12] and cleavage is perfect on [001] and good on [010], with the cleavage planes meeting at an angle of 93 to 94 degrees. [13] It is from this slightly oblique cleavage angle that plagioclase gets its name, Ancient Greek plágios (πλάγιος 'oblique') + klásis (κλάσις 'fracture
The resulting, slightly roughened surface alters the extinction angle of various parts of the crystal lattice, leading to this type of extinction. Since it is not a natural feature of the mineral, bird's eye extinction is not observed in all mica crystals, nor from all angles, but it is quite common, and is used as a diagnostic feature for micas.
2V angle: 65–88° Extinction: Inclined extinction to cleavage: Diagnostic features: Gridiron twinning distinguishes microcline from other feldspars. Distinguishable from plagioclase because the lamellae in plagioclase are continuous and do not "pinch and swell." Alters to: Commonly alters to sericite or clay. Relief: Low negative relief ...
However, for certainty, other distinguishing features of quartz include the fact that it is uniaxial, it has a positive optic sign, length-slow sign of elongation, and zero degree extinction angle. [5] A sigma clast as seen in an ultra thin section. The uneven coloring is an artifact from uneven polishing.
Orthoclase, or orthoclase feldspar (endmember formula K Al Si 3 O 8), is an important tectosilicate mineral which forms igneous rock.The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles to each other.
Anisotropic minerals specifically will show one extinction for each 90 degrees of stage rotation. The extinction angle is the measure between the cleavage direction or habit of a mineral and the extinction. 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 ...
Anorthite is a rare compositional variety of plagioclase. It occurs in mafic igneous rock. It also occurs in metamorphic rocks of granulite facies, in metamorphosed carbonate rocks, and corundum deposits. [3] Its type localities are Monte Somma and Valle di Fassa, Italy. It was first described in 1823. [5]
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula Na Al Si 3 O 8. It is a tectosilicate. Its color is usually pure white, hence its name from Latin, albus. [5]