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The programme looks at why crystal minerals or phenocrysts form in lava and why one molten lava forms phenocrysts of a different mineral from that found in another. Crystallisation and melting beha...viour of mixtures of compounds is also examined.
Metadata describing this Open University video programme
Module code and title: S237, "The Earth, structure, composition and evolution"
Item code: S237; 06
First transmission date: 30-05-1981
Published: 1981
Rights Statement:
Restrictions on use:
Duration: 00:24:25
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Producer: Barrie Whatley
Contributor: John Wright
Publisher: BBC Open University
Keyword(s): Binary eutectic; Diopside; Lava flows; Mineral crystals; Mount Etna; Phase diagram; Phenocrysts; Plagioclase; Ternary eutectic
Footage description: Film shots of a volcano erupting and of cooled lavas on the side of Mt. Etna. John Wright introduces the programme. John Wright points out various different phenocrysts in a sample of basalt from Etna. He goes on the explain that one can simulate the beh aviour of a rock melt by using two organic compounds to represent two minerals. Shots of such an experiment being performed in test tubes. Wright refines the experiment by looking at the behaviour of simulated melts with compositions of various proportions. The results are plotted on a graph and a curve, the eutectic, is derived. The experiment is continued and the now solid samples are re-melted. Wright notes which of the minerals melts first. Wright explains how a phase diagram is constructed and how these are used to understand what happens when rock melts cool. Looking at a phase diagram, he points out the three important features found there, the Liquidus, the Eutectic and the Solidus. Wright uses a phase diagram to show what happens when the melt of a two mineral composition cools. Shots of Wright intercut with the phase diagram. With the aid of models and animations, Wright explains how a three dimensional liquidus surface is constructed which will predict crystallisation of melts with three mineral components. The three dimensional liquidus surface is reduced to a two dimensional phase diagram in which height is represented by equal temperature contour lines Using such a phase diagram for pyroxene, Wright predicts what will happen if a melt of composition 60% Diopside, 25% Anorthite and 15% Albite cools. Wright goes on to predict what will happen to a melt of different composition (60% Anorthite and 40% Albite) as it cools. He points out that cooling must be slow enough for equilibrium to be maintained. Looking at thin section samples of basalt from Etna, Wright uses a phase diagram to work out the conditions under which these, rocks cooled. S267/VCR8
Production number: FOUS163F
Videofinder number: 1593
Available to public: no