Myrmekite under the microscope
One of the more intriguing types of crystallisation is composed of an intergrowth of plagioclase feldspar and quartz, either during the cooling of magmas or during metamorphic or hydrothermal alteration of rocks,. The quartz forms the worm like threads, having grown in igneous varieties simultaneously with the surrounding plagioclase crystal, in a manner somewhat similar to graphic granite (see http://tinyurl.com/njpe4e5) for an illustrated explanation of this fascinating rock). In metamorphic rocks this structure forms when the feldspar is replaced by quartz during metasomatism, the hydration of metamorphosing rocks by hot fluids.
Strictly speaking orthoclase feldspar needs to be present to call it a myrmekite, and the more egenral term for such mineral intergrowths is symplectite. Even in cooling magmas, late stage alteration by the magma's own juices are often the likely cause of myrmekite formation as plagioclase is slowly replaced by orthoclase.
This example is from a high grade metamorphic rock from Italy, and was taken in cross polarised light with the red tint plate inserted. Size of subjects 0.3-10mm.
Image credit: Bernardo Cesare For similar beautiful pictures visit Bernardo's website: www.microckscopica.org http://www.britannica.com/EBchecked/topic/400662/myrmekite https://www2.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000177
