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Rapidly And Accurately Determining
Refractive Indices Of Asbestos Fibers
By Using
Dispersion Staining Method
A Standard Operation Procedure For
Bulk Asbestos Analysis By
Polarized Light Microscopy
Shu-Chun Su, Ph.D.
Rev. 2010-07-11
For those laboratories that have received earlier versions of this paper, please replace them with this updated version. A much condensed version of this paper was published in 2003: A rapid and accurate procedure for the determination of refractive indices of asbestos minerals. American Mineralogist, 88, 1979-1982.
If anyone has any questions or suggestions concerning this procedure or need the electronic files (Word or PDF format), please contact me at shuchunsu@.
This version has corrected an error with the two crocidolite tables (Tables 7 and 8): α and γ tables were transposed in older versions although the resultant differences between the RIs obtained from the previous tables and current tables are in most cases <0.002, which is well within the expected experimental errors resulted from the inherited errors in estimating the matching wavelengths. In fact, because the dispersion coefficients of between α (1.161) and γ (1.174) are so negligible that they were combined into a single table in the above American Mineralogist paper. The same is true for chrysotile and amosite.
Shu-Chun Su, Ph.D.
NVLAP Technical Expert
Bulk and Airborne Asbestos Analysis Programs
Determining Asbestos Refractive Indices by Dispersion Staining Page 37 of 32
Introduction
Refractive index (RI) is the most important optical properties of non-opaque minerals. It is also the leading diagnostic optical property used to identify asbestos components in bulk insulation or building materials by polarized light microscopy (PLM) using oil immersion method (Perkins and Harvey, 1993). Most environmental laboratories in the United States participate in the National Voluntary Laboratory Accreditation Program (NVLAP) administered by the National Institute of Standards and Technology (NIST), U.S. Department of Commerce. NVLAP requires the refractive indices a and g of asbestos fibers to be determined and recorded during routine bulk asbestos sample analysis. Generally, an attainable and reasonable accuracy is "0.005 for chrysotile, amosite, tremolite, actinolite, and anthophyllite, or "0.010 for crocidolite.
In many environmental laboratories, the high volume of samples demands analysts to minimize the amount of time spent on the determination of required optical properties, particularly the refractive indices. It is most desirable to determine both a and g from a single slide or a preparation (Su, 1993). Among the three methods for assessing the direction and magnitude of the mismatch between a solid and a surrounding liquid, Becke line (Bloss, 1961), dispersion staining (McCrone, 1987), and oblique illumination (Stoiber and Morse, 1994), only the later two, i.e., dispersion staining (DS) and oblique illumination (OI), can meet the specific needs for the routine PLM analysis of bulk asbestos samples in commercial environmental laboratories. The advantage of OI method is that it is as simple and accurate as DS and does not require special objective lens. In the meantime, it can be applied using high power objective lens (20X, 40X, etc.)
This paper provides a rapid and accurate procedure to enable bulk asbestos analysts to convert an observed DS color associated with a or g for a specific asbestos mineral in a specific immersion liquid through its corresponding matching wavelength (l0) into corresponding numerical RI value.
Procedure
1. Select a proper immersion liquid to mount the sample
Mount the suspected asbestos fibers in an appropriate liquid according to Table 1. For asbestos types other than chrysotile, there are two choices of immersion liquids. The first choice, which is the liquid outside the parentheses, gives higher accuracy than the second choice, which is the liquid inside the parentheses. For example, when measuring crocidolite, 1.700 liquid is a much better choice than 1.680. For routine analysis, 1.550 (for chrysotile), 1.620 (for tremolite, actinolite, and anthophyllite), and 1.700 (for amosite and crocidolite) are quite adequate to obtain good accuracy for both a and g. When higher accuracy is desirable (for example, when performing NVLAP Proficiency Testing), other liquids may be more appropriate and different liquids may be used for a and g. For example, use 1.615 for the a and 1.635 for the g of anthophyllite. Therefore, additional tables are included for higher accuracy work.
It is imperative to have fresh surface of asbestos fibers in direct contact with the surrounding liquid. Sometimes, the surface of an asbestos bundle may be coated by matrix or binder materials. In this case, true DS colors intrinsic to the asbestos/liquid combination might not be displayed.
Rev. 2010-07-11 (Shu-Chun Su, Technical Expert for NVLAP Asbestos Programs)
Table 1. The Selection of Immersion Liquids for Asbestos Analysis
Suspected Asbestos
Immersion Liquids (Conversion Table Number)
Type
RI
Proficiency Testing Samples
(Different liquids for a and g)
Routine Samples
(Same liquid for both a and g)
Chrysotile
a
1.550 (4A/B)
g
Grunerite
(Amosite)
a
1.680 (5A)
1.700 (6A/B)
[2nd choice 1.680 (5A/B)]
g
1.700 (6B)
Riebeckite
(Crocidolite)
a
1.700 (8A)
1.700 (8A/B) [or 1.680 (7A/B)]
g
Tremolite
a
1.605 (9A)
1.620 (11A/B)
[2nd choice 1.625 (12A/B)]
g
1.6351 (14B)
Actinolite
a
1.6101 (16A)
1.625 (19A/B)
[2nd choice 1.620 (18A/B)]
g
1.6401 (22B)
Anthophyllite
a
1.6151 (25A)
1.625 (27A/B)
[2nd choice 1.620 (26A/B)]
g
1.6351 (29B)
1. Cargille makes two series of immersion liquids in the range of 1.500 to 1.640: Series A (normal dispersion), which is in increment of 0.002, and Series E (high dispersion), which is in increment of 0.005. All oils between 1.605 and 1.640 used to generate the conversion tables in this paper are Series E liquids. For tremolite, actinolite, and anthophyllite, the central-stop DS colors produced by these Series E high dispersion liquids are more intense and vivid than those produced by Series A liquids. Tables 9 to 27 are not applicable if Series A liquids are used instead.
2. For qualitative analysis, 1.605 liquid is somehow okay for tremolite, actinolite, and anthophyllite. When accurate RI measurement is required, 1.605 liquid should be avoided because their g are significantly higher than 1.605 and exhibit yellow to pale yellow central-stop DS colors. The inherent error in converting DS colors to l0 is always higher in the range of yellow than in the range of blue to orange.
A simple and effective way to bring out the true DS colors is to grind or rub the fiber bundle with a needle or probe to break the fiber bundle into finer bundles so that fresh surface is revealed and made in direct contact with the surrounding liquid.
2. Measure the temperature of the immersion liquid
Measure and record t (in °C), the temperature of the immersion liquid on the microscope slide. If the temperature of the liquid, slide, cover glass and sample can be reasonably assumed to be in equilibrium with the room temperature, t can be assumed to be equal to the room temperature. The temperature data is needed for making temperature correction. Certain microscope tends to heat up the slide, resulting in an increase 2° or more in the liquid temperature.
3. Check the alignment of the polarized light microscope
Make sure that the polarized light microscope is properly aligned:
- DS objective and its central stop is centered;
- substage condenser is centered (if possible, set the microscope to Köhler illumination);
- the vibration (or privileged) directions of polarizer and analyzer are parallel to the E-W and N-S cross hairs in the eyepiece, respectively.
4. Observe the central-stop DS color associated with a of the asbestos fibers
Assuming that the polarizer is parallel to the E-W cross hair, rotate the microscope stage until a fiber bundle is parallel to the E-W cross hair if the asbestos is suspected to be crocidolite or perpendicular to the E-W cross hair if the asbestos is suspected to be other five asbestos types (chrysotile, tremolite, actinolite, anthophyllite, and amosite). Although the a of monoclinic amphiboles (tremolite and actinolite) is not exactly perpendicular to the fiber elongation, the RI at this orientation can be assumed to be reasonably close to a. Adjust the aperture diaphragm and field diaphragm to optimize the DS color displayed by the asbestos fibers.
Usually, a range of DS color is displayed. Make sure that the DS color that gives the lowest RI is observed, i.e. the DS color corresponding to the longest l0. For example, if the DS color ranges from blue to light blue, choose light blue.
5. Covert the observed DS color into corresponding matching wavelength, l0, between the asbestos fiber and the immersion liquid used by referring to Figure 1 or Table 2
6. Find out the numerical value of a corresponding to the observed l0 and t
Refer to one of the conversion tables to convert l0 and t into the corresponding refractive index. Notice that each table is for a specific direction (a or g) of a specific asbestos mineral mounted in a specific RI liquid. If an RI liquid with a different nD and/or a different dispersion coefficient [nF-nC] is used, the current tables are no longer applicable. In this case, a new table may be calculated by using an Excel program written by the author, which is available upon request. The algorithm used to compute all conversion tables in this paper can be found in Su (1993 and 2003). The 1993 reference is included in this SOP as an Appendix.
7. Observe the DS color associated with g of the asbestos fibers
Rotate the microscope stage 90° and then repeat Steps 4 - 6 to determine g. Again, a range of DS color is usually displayed. Make sure that the DS color that gives the highest RI is observed, i.e. the DS color corresponding to the shortest l0. For example, if the DS color ranges from purple to red-purple, choose red-purple.
Fig. 1. Converting dispersion staining color to corresponding l0 (McCrone, 1987).
Table 2. Converting dispersion staining color to corresponding l0 (McCrone, 1987)
Matching Wavelength
λ0, nm
Particle Edge Colors1
Becke Line Colors2
Annular Stop3
Central Stop4
Particle
Liquid
<340
Black violet
white
white
X
<400
dark violet
pale yellow
pale yellow
X
430
violet
yellow
pale yellow
X
455
blue
golden yellow
yellow
violet
485
blue-green
orange
orange
violet
520
green
red purple
orange-red
violet-blue
560
yellow-green
purple
red-orange
blue-violet
595
yellow
deep blue
red
blue
625
orange
blue-green
faint red
blue
660
red-brown
light blue-green
X
blue-green
700
dark red-brown
pale blue-green
X
pale blue-green
1500
black-brown
very pale blue-green
X
very pale blue-green
1. In focus
2. On focusing up
3. Observed on a brightfield
4. Observed on a darkfield
Table 3. Refractive Indices and Dispersion Coefficients [nF-nC] of Six Asbestos Minerals
Mineral
nF
nD
nC
[nF-nC]
Reference
Chrysotile
a
1.5563
1.5490
1.5456
0.0107
NIST
SRM 1866
g
1.5649
1.5560
1.5530
0.0119
Grunerite
(Amosite)
a
1.6931
1.6790
1.6734
0.0197
NIST
SRM 1866
g
1.7156
1.7010
1.6951
0.0205
Riebeckite
(Crocidolite)
a
1.7132
1.7015
1.6971
0.0161
McCrone (1987)
Figs. 104A and 104B
g
1.7206
1.7072
1.7032
0.0174
Tremolite
a
1.6128
1.6063
1.6036
0.0092
NIST
SRM 1867
b
1.6299
1.6230
1.6201
0.0098
g
1.6423
1.6343
1.6310
0.0113
Actinolite
a
1.6201
1.6126
1.6095
0.0106
NIST
SRM 1867
b
1.6369
1.6288
1.6254
0.0115
g
1.6485
1.6393
1.6355
0.0130
Anthophyllite
a
1.6227
1.6148
1.6116
0.0111
NIST
SRM 1867
b
1.6350
1.6273
1.6241
0.0109
g
1.6449
1.6362
1.6326
0.0123
1. [nF-nC] is the only parameter used in calculating all conversion tables. When changes in elemental composition, thermal history, etc. have caused variations in nF, nD, and nC, the dispersion coefficient [nF-nC] remains relatively unaffected or only slightly affected.
2. The dispersion coefficient of NIST SRM 1866 grunerite is much higher than that of the grunerite in McCrone (1987, Figs. 104A and 104B). Therefore, some values in Tables 6A and 6B, which are based on NIST grunerite, are markedly different from the values in McCrone (1989, p.51, Table I), which are based on the grunerite in Figs. 104A and 104B (McCrone, 1987).
3. For tremolite, actinolite and anthophyllite, nz is close to a and n2 to g.
Table 4A. Chrysotile a (In Cargille Series E: 1.550)
l0 19°C 21°C 23°C 25°C 27°C 29°C 31°C
400 1.583 1.582 1.581 1.580 1.579 1.578 1.577
420 1.577 1.576 1.575 1.574 1.573 1.572 1.571
440 1.573 1.572 1.571 1.570 1.569 1.568 1.567
460 1.569 1.568 1.567 1.566 1.565 1.564 1.563
480 1.565 1.564 1.563 1.562 1.561 1.560 1.559
500 1.562 1.561 1.560 1.559 1.558 1.557 1.556
520 1.560 1.559 1.558 1.557 1.556 1.555 1.554
540 1.558 1.557 1.556 1.555 1.554 1.553 1.552
560 1.556 1.555 1.554 1.553 1.552 1.551 1.550
580 1.554 1.553 1.552 1.551 1.550 1.549 1.548
589 1.553 1.552 1.551 1.550 1.549 1.548 1.547
600 1.552 1.551 1.550 1.549 1.548 1.547 1.546
620 1.551 1.550 1.549 1.548 1.547 1.546 1.545
640 1.549 1.548 1.547 1.546 1.545 1.544 1.543
660 1.548 1.547 1.546 1.545 1.544 1.543 1.542
680 1.547 1.546 1.545 1.544 1.543 1.542 1.541
700 1.546 1.545 1.544 1.543 1.542 1.541 1.540
750 1.544 1.543 1.542 1.541 1.540 1.539 1.538
800 1.542 1.541 1.540 1.539 1.538 1.537 1.536
Table 4B. Chrysotile g (In Cargille Series E: 1.550)
l0 19°C 21°C 23°C 25°C 27°C 29°C 31°C
400 1.581 1.580 1.579 1.578 1.577 1.576 1.575
420 1.575 1.574 1.573 1.572 1.571 1.570 1.569
440 1.571 1.570 1.569 1.568 1.567 1.566 1.565
460 1.567 1.566 1.565 1.565 1.564 1.563 1.562
480 1.564 1.563 1.562 1.561 1.560 1.559 1.558
500 1.562 1.561 1.560 1.559 1.558 1.557 1.556
520 1.559 1.558 1.557 1.556 1.555 1.554 1.553
540 1.557 1.556 1.555 1.554 1.553 1.552 1.551
560 1.555 1.554 1.553 1.552 1.551 1.550 1.549
580 1.554 1.553 1.552 1.551 1.550 1.549 1.548
589 1.553 1.552 1.551 1.550 1.549 1.548 1.547
600 1.552 1.551 1.550 1.549 1.548 1.547 1.546
620 1.551 1.550 1.549 1.548 1.547 1.546 1.545
640 1.550 1.549 1.548 1.547 1.546 1.545 1.544
660 1.548 1.547 1.546 1.546 1.545 1.544 1.543
680 1.547 1.546 1.545 1.
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