Search results
Results from the WOW.Com Content Network
Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or "record-groove" effect. Plagioclase is a major constituent mineral in Earth's crust and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks.
Closely spaced polysynthetic twinning is often observed as striations or fine parallel lines on the crystal face. Cyclic twins are caused by repeated twinning around a rotation axis. This type of twinning occurs around three, four, five, six, or eight-fold axes, the corresponding patterns are called threelings, fourlings, fivelings , sixlings ...
Like others of the series, bytownite forms grey to white triclinic crystals commonly exhibiting the typical plagioclase twinning and associated fine striations. The specific gravity of bytownite varies between 2.74 and 2.75. The refractive indices ranges are nα=1.563 – 1.572, nβ=1.568 – 1.578, and nγ=1.573 – 1.583.
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]
Microcline (cross-hatched twinning) included in magnetite (black, opaque) in plagioclase (polysynthetic twinning). Therefore, the sequence of formation is: microcline → magnetite → plagioclase. Peak porphyroblasts: A garnet-mica schist with porphyroblastic garnet (black) in fine-grained mica matrix: Retrograde (Post-peak) reaction rims
Crystal twinning is common in feldspars, especially polysynthetic twins in plagioclase and Carlsbad twins in alkali feldspars. If the latter subgroup cools slowly from a melt, it forms exsolution lamellae because the two components – orthoclase and albite – are unstable in solid solution.
Labradorite is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573 and twinning is common.
The ratio of alkali feldspar to plagioclase feldspar, together with the proportion of quartz, is the basis for the QAPF classification of igneous rock. [ 15 ] [ 16 ] [ 17 ] Calcium-rich plagioclase is the first feldspar to crystallize from cooling magma, then the plagioclase becomes increasingly sodium-rich as crystallization continues.