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Compact Course Microscopy of rock-forming Minerals

Compact Course Microscopy of rock-forming Minerals Part 3 : Cordierite, Apatite, Nepheline, Leucite. Cordierite Formula : (Mg,Fe) 2 Al 2 Si[Al 2 Si 4 O 18 ] Sy m metry : orthorombic n : 1,530 – 1,578 n : 0,008 – 0,018 2V x : 42° – 104° max. I. F. (30 μ m) : white to red I.

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Compact Course Microscopy of rock-forming Minerals

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  1. Compact Course Microscopy of rock-forming Minerals Part 3 : Cordierite, Apatite, Nepheline, Leucite

  2. Cordierite Formula : (Mg,Fe)2Al2Si[Al2Si4O18] Symmetry : orthorombic n : 1,530 – 1,578 n : 0,008 – 0,018 2Vx : 42° – 104° max. I. F. (30μm) : white to red I Observations: Important mineral in many Al-rich metamorphic rocks (up to amphibolite facies, not to very high pressures). Also in Al-rich (S-type) granites and rare in volcanic rocks. Typical mineral of contact metamorphic rocks in Al-rich sediments (slates and shists) Retrogressive alteration to fine-grained chlorite (pinitization)

  3. 0,5 mm Cordierite • Observations: • Cordierite • trace of twinning • pleochroic halos

  4. 0,5 mm Cordierite, three individual crystals intergrown in triplet along (110)

  5. 0,1 mm Cordierite, pleochroic halos Observation: Pleochroic halos form -radiation from radioactive of Uranium, Thorium and REE. These elements are concentrated in small mineral inclusions such as zircon, monazite or xenotime). Different halo diameters are either due to 3D effects in cutting or different energies of decay (and thus different elements decaying) Watch out, next slide will turn the polarizer

  6. 0,5 mm Cordierite, incipient pinitisation Observations: Areas with pinitisation appear to have higher relief !

  7. Observations: • Relicts of Cordierite • Areas with pinitisation appear to have higher relief ! 1 mm Cordierite, almost complete pinitisation

  8. 0,5 mm Cordierite, Fibrolith, orientiented elongated inclusions of Sillimanite

  9. Observations: • Sillimanite elongated and oriented parallel to plane of section • Sillimanite vertical to plane of section 0,2 mm Cordierite, Fibrolith, Detail

  10. Note twinning ! 0,5 mm Cordierite,Porphyroblasts in contact metamorphic Observations: Turbid crystals due to abundant fine inclusions.

  11. Hexagonal when cut c Apatite Formula : Ca5[(F,Cl,OH)|PO4)3] Symmetry : hexagonal n : 1,631 – 1,667 n : 0,003 – 0,005 2V : - max. I. F. (30μm) : grey I Observations: Apatite contains most of the P in a rock, thus it is a widely distributed mineral in many rock types. Easy to recognize by its moderately high refractive index and low birefringence (grey I). Shape prismatic along c axis, normal extinction, negative character of c. Cl-Apatit

  12. Hexagonal when cut c. Isotropic indicatrix, uniaxial negative 0,5 mm Apatite Observations: • Elongated when cut //c. Interference colour grey. Negative with compensator (turns blue with red I) • Note ! Easy to recognise and abundant, serves as a reference for refractive index relative to other • unknown • minerals.

  13. 0,2 mm Apatite • Observationsn: • cut vertical c, • isototropic. • elongated, cut // c, • grey I

  14. 100 μm Apatite • Observations: • small apatite inclusions, short and prismatic crystals.

  15. Nepheline Formula : (Na,K)[AlSiO4] Symmetry : hexagonal n : 1,529 – 1,547 n : 0,003 – 0,005 2V : - max. I. F. (30μm) : grey I Observations: Important mineral in silica undersaturated rocks (foid). Typical are low refractive index andlow birefringence!

  16. Observationsn: • elongated, cut // c, • grey I 1 mm Nepheline • Observations: • cut vertical c, • isototropic. • elongated, cut // c, • grey I • Typical short prismatic crystals, but also often xenomorphic in interstices

  17. 1 mm Nepheline • Observations: • Nepheline colourless in interstices • cut vertical c • cut // c, grey I

  18. Leucite Formula : K[AlSi2O6] Symmetriy : tetragonal n : 1,508 – 1,509 n : 0,001 2V : very small max. I. F. (30μm) : dark grey I Observations: Important foid in silica undersaturated rocks. Always paramorphs of tretrahedral leucite after cubic leucite. Crystal shape cubic deltoidikositetraeder {210}. Paramorphic replacement of high-leucite (cubic) by low leucite (tetragonal) results in twinning and crossed interfrerence patterns. Only in volcanic rocks, always idiomrophic

  19. 0,5 mm Leucite Observations: Paramorphic replacement of high-leucite (cubic) by low leucite (tetragonal) results in twinning and crossed interfrerence patterns. Deltoidikositetraeder

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