Olivine thin section microscopy under the Zeiss IM microscope with crossed polarisation 8/7/2017

Many meteorites contain olivine – the following are pictures from my slide collection of olivine from Earth showing birefringence as seen in meteorite samples. The slide I have comes from Knocksghey Sill, North Antrim.

Meteorite minerals contain nickel and iron in their native state (not combined with oxygen), which are rarely found in terrestrial rocks. In addition, they comprise most of the minerals in “iron meteorites.” The most common minerals are pyroxenes followed by plagioclase feldspar, and olivine (https://nau.edu/cefns/labs/meteorite/about/meteorite-minerals/). Olivine is magnesium iron silicate.

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent/birefractive (https://en.wikipedia.org/wiki/Birefringence).

The Maidens (Irish: Na Maighdeanacha) or Hulin Rocks (Irish: Na Faoilinn, meaning “the seagulls”) are two islets and several skerries in the North Channel off County Antrim in Northern Ireland. The Eastern Maiden or Southern Rock lies about 9 km from the coast at Ballygalley, or 13 miles from Larne. The West Maiden or Northern Rock is about half a mile further out. Lighthouses were built on both rocks; the West Maiden was abandoned in 1903 and the East Maiden was automated in 1977.┬áThe Maidens is an example of an olivine-dolerite sill, one of many similar intrusions along the Antrim coast. They were formed during the Paleogene and Neogene periods (66 million to 2.6 million years ago), a period of intense volcanic activity in Northern Ireland. The Giant’s Causeway, also from this time period, was created by volcanic eruption. At the Maidens, magma was forced underground through layers of weakness, but never reached the surface. It cooled more slowly, solidifying and leaving a hard rock, relatively resistant to weathering compared to the strata around it and so forming the islands left today. Other examples of nearby sills are Fair Head, the Portrush sill which forms Ramore Head and the Skerries, and the Knocksoghey Sill near Ballintoy, part of which forms Sheep Island (https://en.wikipedia.org/wiki/The_Maidens)

Andy

 

Olivine-Dolerite-4x-objective-crossed-polarisation-080717I.bmp (below – from Earth not meteorite origin):

Twisting one of polar filters (below):

Olivine-Dolerite-4x-objective-crossed-polarisation-080717III.bmp (below):

Olivine-Dolerite-32x-objective-crossed-polarisation-080717I.bmp (below):

Olivine-Dolerite-32x-objective-crossed-polarisation-080717II.bmp (below):

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