guide to thin section microscopy - Mineralogical Society of America

More documents

Recommendations

Info

Guide to Thin Section Microscopy Conoscopy Determining the optic sign of biaxial minerals For the determination of the optic sign, crystal sections with lowest-possible interference colours should be selected. For minerals with low 2V, acute bisectrix figures are also suitable (Fig. 4-54). Appropriate grains will show low, but not zero birefringence in orthoscopic mode. In general, it is sufficient to see one optic axis (melatope) within the field of view. Ideally, the point of maximum curvature (melatope) of the isogyre should lie in the centre of the field of view, such that the direction of curvature can be seen clearly. The convex side of the hyperbola points to the acute bisectrix, the concave side to the obtuse bisectrix. For the observation of the change of interference colour when a compensator is inserted, the isogyres should be positioned such that the optic axial plane is in diagonal, NE-SW orientation. In this orientation, the concave sides of the isogyres point NE and SW, or either direction, if only a single isogyre is in the field of view. After inserting the first-order red plate, the isogyres appear in 1 st -order red, while the interference colours in between the isogyres change in accordance with the optic sign of the mineral (Fig. 4-54): Case A: On the convex sides of the isogyres subtraction occurs (1 st -order orange-yellow close to the melatope), while the concave domains show addition (2 nd -order blue close to the melatope). The acute bisectrix is Z; the optic sign of the minerals is therefore positive (Fig. 4-54; example aragonite). Case B: On the convex sides of the isogyres addition occurs (2 nd -order blue), while the concave domains show subtraction (1 st -order orange-yellow). The acute bisectrix is X; the optic sign of the minerals is therefore negative (Fig. 4-54; example muscovite). Raith, Raase & Reinhardt – February 2012 Figure 4-54. Determination of the optic sign of a biaxial crystal. The optic sign can be determined in sections perpendicular to the acute bisectrix or perpendicular to one of the two optic axes. The distinction between positive and negative optic sign is made by observing the addition and subtraction phenomena resulting from insertion of the first-order red plate. The isogyres of the mica show distinct blue fringes of their concave side due to dispersion of the optic axes. 123
Guide to Thin Section Microscopy Conoscopy When studying high-birefringent minerals, it is suggested that the quartz wedge is used instead of the first-order red plate. When inserting the quartz wedge, the isochromes in the four quadrants migrate in a characteristic way (Fig. 4-55). Raith, Raase & Reinhardt – February 2012 Figure 4-55. Determination of the optic sign of anisotropic minerals 124
Page 1 and 2:
GUIDE TO THIN SECTION MICROSCOPY Se
Page 3 and 4:
Guide to Thin Section Microscopy Co
Page 5 and 6:
Guide to Thin Section Microscopy Pr
Page 7 and 8:
Guide to Thin Section Microscopy Te
Page 9 and 10:
Guide to Thin Section Microscopy Mi
Page 11 and 12:
Page 13 and 14:
Page 15 and 16:
Page 17 and 18:
Page 19 and 20:
Page 21 and 22:
Page 23 and 24:
Page 25 and 26:
Page 27 and 28:
Page 29 and 30:
Page 31 and 32:
Guide to Thin Section Microscopy Me
Page 33 and 34:
Page 35 and 36:
Page 37 and 38:
Page 39 and 40:
Guide to Thin Section Microscopy Cr
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Guide to Thin Section Microscopy Cl
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
Guide to Thin Section Microscopy Tw
Page 57 and 58:
Guide to Thin Section Microscopy Tw
Page 59 and 60:
Guide to Thin Section Microscopy In
Page 61 and 62:
Guide to Thin Section Microscopy Ex
Page 63 and 64:
Guide to Thin Section Microscopy Re
Page 65 and 66:
Guide to Thin Section Microscopy Al
Page 67 and 68:
Guide to Thin Section Microscopy Op
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80: Guide to Thin Section Microscopy Co
Page 85 and 86: Guide to Thin Section Microscopy Li
Page 87 and 88: Guide to Thin Section Microscopy Do
Page 107 and 108: Guide to Thin Section Microscopy Ex
Page 129: Guide to Thin Section Microscopy Co
show all

guide to thin section microscopy - Mineralogical Society of America

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?