guide to thin section microscopy - Mineralogical Society of America

More documents

Recommendations

Info

Guide to Thin Section Microscopy Double refraction Interference colours: In contrast to monochromatic light, the use of white light provides a full spectrum of wavelengths (spectral colours) which, for a given retardation, is modified in the analyzer through interference such that certain wavelengths are transmitted at full intensity; others are reduced to a variable degree or are obliterated entirely. White light exiting a colourless anisotropic crystal comprises an infinite number of wave couples corresponding to all spectral colours, each wavelength represented by a wave couple with mutually orthogonal vibration directions that are fixed by the crystal's orientation (cf. Figs. 4-25A and B which show such single wave couples). Interference in the analyzer between the two waves of each couple across the light spectrum is a way of making retardation visible and also quantifiable. Any specific retardation modifies the full white-light spectrum, generating a characteristic wavelength spectrum and wave amplitude pattern, which in combination produce a unique interference colour. Evidently, interference colours can only be generated from polychromatic light. They correlate directly with retardation. The sequence of interference colours in relation to increasing retardation is explained graphically in Figs. 4-27 and 4-28. Raith, Raase & Reinhardt – February 2012 Figure 4-27. Constructive and destructive interference as a function of wavelength λ and retardation Γ in the spectral range of visible light (see also Fig. 4-28). The variation of the intensity of the light being transmitted by the analyzer is described by the equation I = 100[sin 2 (Γ/λ)180°]. To give a few examples: In the lower range of retardation (Γ = 0 ↔ 200 nm) black and grey colour tones dominate as the complete colour spectrum of white light is affected by a reduction of intensities. At around Γ = 250 nm only wavelengths from both ends of the spectrum are slightly diminished which is perceived as close to white ("first-order white"). The range Γ = 400 ↔ 500 nm shows characteristic orange to light red interference colours, as blue and green wavelengths are suppressed, while longer wavelengths dominate the spectrum. This situation is reverse in the range Γ = 600 ↔ 650 nm. Here, the shorter wavelengths dominate, which results in a blue interference colour. The distinctive purple colour at Γ = 551 nm ("first-order red") lies in a position where the intermediate wavelengths (green to orange) 89
Guide to Thin Section Microscopy Double refraction Raith, Raase & Reinhardt – February 2012 Figure 4-28. Graphical presentation explaining the sequence of interference colours. The interference colour for a specific value of Γ derives from the sum of all wavelength amplitudes in the visible spectrum that have been modified by interference in the analyzer (or, in other words, the sum of intensities of all wavelengths across the spectrum after interference in the analyzer). As can be derived from the figure, a specific interference colour in its precise spectral composition cannot occur more than once in the colour chart (cf. Fig. 4-27). 90
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: Guide to Thin Section Microscopy Cr
Page 47 and 48: Guide to Thin Section Microscopy Cl
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 75 and 76: 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 95: Guide to Thin Section Microscopy Do
show all

guide to thin section microscopy - Mineralogical Society of America

Create successful ePaper yourself

Delete template?

Save as template?