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Lab #7: Identifying Minerals
This is the ability to quickly determine and assess the properties of a mineral in PPL and XPL, and from these properties and the list of minerals known to occur in the general category of rock, to identify the mineral without compiling a list of observed properties. Eventually in a quick examination, you will be able to identify and list all of the minerals in a thin section; occasionally an unknown mineral will require additional work (or will not be able to be identified). Most of the time examining a thin section should be spent observing and recording textural information.
Preliminary examination of minerals in a thin section:
1) LOW magnification and PPL: opaque or transparent, colour and pleochroism, shape of the mineral (HABIT), anhedral-subhedral-euhedral, cleavage and fracture.
2) LOW magnification and XPL: isotropic or anisotropic, birefringence, extinction angle, twinning and zoning, anomalous extinction, alteration
3) MEDIUM magnification and PPL: relative refractive indices and relief
4) HIGH magnification and conoscopic illumination: uniaxial or biaxial, optic sign, 2V
Mineral properties that are worthy of a description:
a) mineralogically important optical properties that allow you to tell the composition within a solid solution series; e.g. pleochroism colours of amphibole
b) composition, zoning, twinning, and exsolution textures of plagioclase
c) twinning and exsolution features of potassic feldspar
d) zoning and exsolution features of other minerals such as pyroxenes, tourmaline
e) shape, euhedral..anhedral, and number of rational faces
f) size, with a range and an average
g) primary textural and fabric relationships of all minerals
h) any superimposed fabric and textural elements such as recrystallization
i) alteration of primary minerals, including degree of alteration and secondary minerals
j) any changes in mineralogy, texture, or structure across the thin section- not including the common thinning of the rock at the edge of the slide, and not including air pockets/bubbles
Step 1: Using thin section MIN3 and L1698 (with the known minerals that are listed for them), confirm each of the minerals present with your TA. This is to ensure you are correctly identifying the minerals in thin section.
Step 2: Complete the following-
- 1) Thin Section #25: List the minerals present and their modal abundances in a table. Provide a summary of each mineral phase with identifying features present (not all of the ID features, just the ones that were important in identifying the mineral in this particular thin section – like the ones listed above!)
- 2) Thin Section “Olivine Basalt”: List the minerals present and their modal abundances in a table. Provide a summary of each mineral phase with identifying features present.
- 3) Thin Section L121: List the minerals present and their modal abundances in a table. Provide a summary of each mineral phase with identifying features present.
- 4) Thin Section 1415: List the minerals present and their modal abundances in a table. Provide a summary of each mineral phase with identifying features present.
- 5) Thin Section L1888 or L1889: What is the highly altered mineral present?
- 6) Thin Section 402: Identify the isotropic mineral.
- 7) Thin Section “D”: Identify the green mineral present.