Rocks and minerals: Igneous rocks 2.1.2
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|(a)||i) the classification of igneous rocks on the basis of their composition (silicic, intermediate, mafic and ultramafic) and crystal grain size (coarse-crystals >5 mm diameter; medium-crystals 1-5 mm diameter; fine-crystals <1 mm
(ii) the diagnostic properties of rocks to identify igneous rocks in samples, photographs and thin section diagrams
|To,include granite, microgranite, rhyolite; diorite, microdiorite, andesite; gabbro, dolerite, basalt and peridotite|
|(b)||i) igneous textures, crystal size and crystal shape as evidence for depth of formation and rate of cooling of igneous rocks
(ii) the diagnostic properties of igneous textures and crystal shape in samples photographs and thin section diagrams
|To include volcanic and plutonic igneous rocks
To include: equicrystaline, glassy, vesicular, amygdaloidal, flow banding and porphyritic
Three videos showing the effect of cold, room temperature and warm microscope slides on the crystallisation of salol.
The fast and medium cooling rates are very clear in terms of crystal size and growth rate.
Learners can begin to apply what they have learnt in the previous topic by considering how igneous rocks are classified based on their chemical composition and crystal grain size, learners could build this information into a table. Using a range of hand specimens, photographs and thin sections learners should be introduced to the range of textures including crystal size and shape and be guided through how these are identified, measured and recorded. A range of online resources may help learners to understand how to sketch these textures by demonstrating good practice.
Common misconceptions or difficulties students may have:
Where learners my struggle in this topic is in taking time to concentrate and look at detailed observations in thin sections and hand specimens and in knowing how to use hand lenses and microscopes effectively. Learners will need time to develop these skills and should be given the opportunity to regularly revisit these methods over the course. Examples of good sketching and observations should be shared.
Students are often reluctant to engage with actual rocks which then can lead to misconceptions in their understanding which may then lead to problems in assessments. Developing the confidence to identify rocks that they encounter in their everyday lives is a real breakthrough moment for students and a great motivator.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course:
This topic is revisited throughout the course, the main section it relates to is igneous processes at plate boundaries (3.2.2) where learners consider magma generation in different plate settings and igneous petrology (5.3) where learners investigate the science behind the melting and crystallisation process.
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