perthitic


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perthitic

[pər′thid·ik]
(geology)
Of a texture produced by perthite, exhibiting sodium feldspar as small strings, blebs, films, or irregular veinlets in a host of potassium feldspar.
References in periodicals archive ?
The sample is a granular, medium-grained, pink granite composed mainly of quartz grains; perthitic potassium feldspar grains up to 0.75 cm in diameter, which are moderately altered to very fine-grained sericite along fractures or twinning planes; plagioclase grains that range from small inclusions hosted by potassium feldspar to moderately large subhedral-euhedral grains with a maximum diameter of 0.5 cm are locally altered to sericite along albite twin planes; and iron-rich biotite laths, which are altered to chlorite along cleavage planes.
The alkali feldspar, including microcline, is mostly perthitic and occurs as megacrysts that occasionally display simple twinning (Figures 3(a) and 3(b)).
Perthitic and myrmekitic intergrowths are observed in few sections.
Its amount increases abruptly close to the contacts of the anorthosites with silicic rocks, where perthitic alkali feldspar partly replaces plagioclase.
K-feldspar is slightly perthitic. Accessory phases are apatite, zircon, ilmenite and magnetite.
Detailed field and petrographic studies reveal that Jura granitoids are megaporphyritic in nature containing phenocrysts of plagioclase and perthitic feldspar.
It consists of quartz, K-feldspar (perthitic orthoclase), and plagioclase in approximately equal proportions, with minor biotite and hornblende.
Some K-feldspar phenocrysts are perthitic. Topaz occurs in the form of irregular grains and/or anhedral aggregates, rimmed by a mixture of white mica and clay minerals (dickite) (Fig.
The cavities are lined with euhedral crystals of smoky quartz, greenish perthitic orthoclase ..., blades of dark bottle-green biotite, and crystals of beryl (aquamarine) and topaz.
Also among the basement rocks is the Devonian Petit-de-Grat Granite containing a distinctive perthitic texture (White et al.
The granite is generally grey to grayish white, essentially composed of perthitic feldspar, quartz and small amount of plagioclase and sodic minerals such as riebeckite and aegirine.
Quartz (~ 30%) and perthitic K-feldspar (~ 60%) with subordinate albite grains is the main constituent indicating the hypersolvus character, alkali amphiboles (Table 3) in the amount of around 10 % are the only mafites, mica minerals are practically absent.