Department of Transportation
for Road and Bridge Construction
NJDOT A-2 – Determining Percentage of Mica in Fine Aggregate
This test method is used to determine the mica content of fine aggregate.
- Apparatus. Use the following apparatus:
- Binocular microscope (Leica, Zeiss, Wild, or equivalent) with 20× minimum magnification and using a standard 10× high
- No. 10 and No. 100 sieves conforming to AASHTO M 92.
- Mechanical sieve shaker conforming to AASHTO T 27.
- Microsplitter or spinning riffler splitter.
- Balance conforming to AASHTO M 231 and having a minimum capacity of 100 grams with a precision of 0.1 gram.
- Balance for weighing separated mica in stainless steel containers having a minimum capacity of 50 grams with a precision of 0.001 grams and conforming to AASHTO M 231.
- Stainless-steel sample container with lid.
- Oven for drying sample to a constant weight at a temperature of 230 ± 9 °F.
- Stainless-steel spatula.
- Size 000 or smaller paintbrush with loose hairs trimmed.
- Microscopic picking tray as used by the American Association of Petroleum Geologists or equivalent for placement within microscope working area.
- Water in a suitable container for wetting paintbrush.
- Various suitable larger brushes for moving and spreading sample on picking tray.
- Procedure. Perform the following steps:
- Ensure that the examiner has a minimum 4-year degree in geology or a related science and an understanding of mineralogy, petrographic analysis, and optical microscopy.
- Take the test sample from a representative sample from a field stockpile. After oven drying to a constant mass at a temperature of 230 ± 9 °F, split the sample into approximately 200 grams weighed to the nearest 1 gram.
- Sieve the 200 gram sample according to AASHTO T 27 using a No. 10 sieve and a No. 100 sieve. Retain the material passing the No. 10 sieve and retained on the No. 100 sieve.
- Reduce the retained sample to approximately 20 grams. Using a microsplitter or a spinning riffler further split the sample into 20 approximately 1-gram samples of fine aggregate. Choose 2 of the twenty 1-gram samples at random for analysis.
- Place the first chosen sample on a microscopic picking tray and quarter using a stainless-steel spatula. With a large brush, sweep 3 of the 4 sample aliquots to the periphery of the picking tray for possible further analysis.
- Wet the bristles of the small paintbrush and under 10-20X viewing power, separate the mica minerals from the rest of the fine aggregate
- After picking at least 95 percent of the mica from the sample aliquot, carefully brush the separated mica into pre-weighed and labeled stainless-steel sample container with lid. Brush the mica-free portion of the sample aliquot into another pre-weighed and labeled sample container with lid.
- If deemed necessary, the examiner may analyze other quarter aliquots of the sample brushed aside in Step 5. Follow Steps 6 and 7 to analyze additional aliquots brushing the mica and the mica-free portions into their respective containers with the first aliquot analyzed.
- With the lids off, dry both portions of the sample to constant weight.
- Using a scale with a precision of 0.001 grams, weigh both samples with the lids on to prevent loss. Record weights of the mica and the mica-free portions. The sum of these is the “Weight of Sample” and should be approximately 0.25 grams for each aliquot analyzed.
- Repeat Steps 5 through 10 for the second randomly chosen 1-gram sample.
- Calculations and Report. Calculate the percent of mica for each sample analyzed using the following equation:
| Percent of Mica =
Weight of Mica in Grams
|| x 100
Weight of Sample
- Report. Report the final mica content as the average of the results of the 2 samples to the nearest 0.1 percent.
Last Document Correction:
December 14, 2007