There is also some variation in the amount of quartz present, and in the relative importance of the orthoclase and plagioclase.
Sometimes 584 the striae on crystal-faces are due to repeated lamellar twinning, as in the plagioclase felspars.
The former consist mainly of plagioclase, leucite and augite, while the latter contain olivine in addition.
Many dolerites are porphyritic and carry phenocrysts of olivine, augite and plagioclase felspar (or of one or more of these).
The felted or hyalopilitic texture is the most common, and plagioclase is the principal groundmass constituent.
More rarely biotite and augite exhibit the same relations to the plagioclase.
The plagioclase crystals vary greatly in size, some being mere microliths while others are over the eighth of an inch in length.
Their essential minerals are pyroxene, plagioclase and garnet: chemically they resemble the gabbros.
Minettes contain biotite and orthoclase; kersantites, biotite and plagioclase.
The finished implements found were two axes, of a plagioclase igneous rock, and three arrow points, all of quartzite.
Any of a series of common feldspar minerals, consisting of differing mixtures of sodium and calcium aluminum silicates. Plagioclase is typically white, yellow, or reddish-gray, but it can also be blue to black. It occurs in igneous rocks. The minerals albite, oligoclase, andesine, labradorite, bytownite, and anorthite are all plagioclases. Chemical formula: (Na,Ca)Al(Si,Al)Si2O8.