| 21. | Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars.
|
| 22. | The " orthoclase sunstone " variant has been found near Amelia Courthouse, Amelia County, Virginia.
|
| 23. | The feldspar ranges from bytownite to oligoclase, being usually a variety of labradorite; orthoclase is scarce.
|
| 24. | The orthoclase crystals are interpreted to represent rock fragments that became enveloped during hardening of the dacitic lavas.
|
| 25. | This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate.
|
| 26. | For example, orthoclase is typically feldspar from granite, while its modifications occur in lavas of similar composition.
|
| 27. | The feldspar of the granulites is mostly orthoclase or cryptoperthite; microcline, oligoclase and albite are also common.
|
| 28. | Higher than normal potassium favors the production of orthoclase, which is usually absent from most mafic igneous rocks.
|
| 29. | Of these the former consists of orthoclase, nepheline, sodalite, diopside and aegirine, biotite and sphene.
|
| 30. | The granodiorite has large crystals of biotite, hornblende, and both feldspars & mdash; orthoclase and plagioclase.
|
