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This programme introduces the study of rocks in hand specimen and in thin section. The speakers are Professor Ian Gass, Professor in Earth Sciences at the Open University, John Wright, Reader in Ea...rth Sciences and Pr. Chris Wilson, Senior Lecturer in Earth Sciences. John Wright shows some examples of minerals; (quartz, mica and feldspar) and shows that rocks are essentially mixtures of minerals. The textures of various rocks are examined in closeup. Two pieces of polaroid and a light source are used to demonstrate how a petrological microscope works. Several rocks are examined in thin section down a microscope to show various optical properties such as the four extinction positions in quartz, pleochroism and straight extinction in biotite, the isotropic behaviours of garnet. Then the textures of the rocks seen in hand specimen are re-examined in thin section. They are the igneous texture of granite, the preferred orientation of mica schist, and the gragmental texture of a sandstone. Chris Wilson discusses the importance of field relations in the study of rocks. He examines two rocks which he collected in Northern Ireland, one is a basalt and the other a sandstone. He is able to deduce the conditions under which these rocks formed, and concludes that they are able to tell us quite a lot about the geological history of the area.
Metadata describing this Open University video programme
Item code: S23-; 01
First transmission date: 22-01-1972
Published: 1972
Rights Statement:
Restrictions on use:
Duration: 00:22:23
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Producer: Jim Stevenson
Contributors: Ian Gass; John Wright
Publisher: BBC Open University
Keyword(s): Feldspar; Field relations; Hand specimen; Mica; Northern Ireland; Optical properties; Petrological microscope; Quartz; Rocks; Thin section
Footage description: Ian Gass introduces the programme John Wright with 3 mineral specimen - mica, feldspar and quartz. He points out their contrasting properties. Tie points out that minerals like these are aggregated together to form rocks. Wright places a granite specimen under a close up camera and points out the crystal minerals. John Wright discusses the importance of rock texture. He compares a metamorphic crystalline rock with the granite. Both are made of the same minerals but have a different texture. They are then compared with a fragmented rock. John Wright with a MacArthur polarising microscope. He explains briefly how rock specimen are prepared. Wright briefly explains how the microscope works using a model of the microscope's polarising mechanism. Quartz crystal in thin section is shown under the microscope to demonstrate the distinction between isotropic and anisotropic minerals. The four extinction positions of anisotropic minerals are shown. A biotite thin section is examined under the microscope to demonstrate the property of pleochroism, a differential absorption phenomenon viewed with analyser out. It is demonstrated that pleochroic substances, being anisotropic, have four extinction phases. A garnet thin section is viewed through a complete stage rotation. Opaque minerals are examined with analyser both in and out. They remain dark in both cases. Wright next examines specimen of igneous, metamorphic and fragmental rock under the microscope for texture. Ian Gass briefly emphasises the importance of field studies in assessing the geological history and identity of a given rock. He introduces R.C.L. Wilson. Chris Wilson with two rock specimen collected from Northern Ireland. He examines both with a hand lens and demonstrates that the light coloured specimen is clearly a fragmental rock while not much can be determined about the dark coloured one by this method. Chris Wilson examines thin sections from both specimen under the microscope. He identifies the dark specimen as a basalt and determines that the fragmental rock was formed by erosion from metamorphic rock. Chris Wilson with photographs of Northern lreland points out where the specimen were collected. Tie concludes thn progrzunme by re-emphasising the value of field data in any attempt to identify rocks correctly and to interpret their history reliably.
Master spool number: 6LT/70259
Production number: 00521_2150
Videofinder number: 1708
Available to public: no