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Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less potassium feldspar. Quartz is present at between 5 and 20% of the rock. Biotite, amphiboles and pyroxenes are common dark accessory minerals. [1] Quartz diorite occurs in association with other granitic rock such as granodiorite, and with volcanic rock. In ...
Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites appear, in outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue, or grey, and may exhibit an iridescence known as labradorescence on fresh surfaces. The feldspar variety labradorite is commonly present in anorthosites.
Monzonite is defined as rock having less than 5% quartz in its QAPF fraction and in which alkali feldspar makes up between 35% and 65% of the total feldspar content. If quartz constitutes greater than 5% of the QAPF fraction, the rock is termed a quartz monzonite , while if feldspathoids are present as up to 10% of the QAPF fraction, the rock ...
The most common mineral assemblage of granulite facies consists of antiperthitic plagioclase, alkali feldspar containing up to 50% albite and Al 2 O 3-rich pyroxenes. Transition between amphibolite and granulite facies is defined by these reaction isograds: amphibole → pyroxene + H 2 O biotite → K-feldspar + garnet + orthopyroxene + H 2 O.
Plagioclase displaying cleavage. (unknown scale) In volcanic rocks, fine-grained plagioclase can display a "microlitic" texture of many small crystals. Plagioclase (/ ˈ p l æ dʒ (i) ə ˌ k l eɪ s, ˈ p l eɪ dʒ-,-ˌ k l eɪ z / PLAJ-(ee)-ə-klayss, PLAYJ-, -klayz) [4] is a series of tectosilicate (framework silicate) minerals within the feldspar group.
A poikilitic texture is most easily observed in petrographic thin sections. In some rocks there seems to be little tendency for the minerals to envelop one another. This is true of many gabbros, aplites and granites. The grains then lie side by side, with the faces of the latter moulded on or adapted to the more perfect crystalline outlines of ...
Micrographic differs from graphic granite in being so much finer-grained that the texture can only be seen in a petrographic thin section with a microscope. The feldspar is usually orthoclase , but can also be albite , oligoclase or microcline .
Higher color indices correlate with higher plagioclase to alkali feldspar ratios. [7] The most common high color index mineral in an S-type granite is biotite. [1] [4] Figure 7. Cross-polarized light photomicrograph of sample CV-114 from the S-type Strathbogie Granite with quartz and feldspar displaying a granophyric texture.