ASTM International

Standard Classification for Natural Muscovite Block Mica and Thins Based on Visual Quality
 N D351

Annotation

This classification covers the determination of commercially available natural muscovite block mica and is intended to be independent of the basic color of the mica or its source.

Muscovite mica is characterized by having an optical axial angle between 50 and 75° (see Appendix X1); and has a weight loss when heated for 5 min at 600°C not exceeding 0.2 % (based on the weight after drying at 120°C).

The visual system of classifying the quality of natural muscovite mica covered by this specification is based upon relative amounts of visible foreign inclusions such as air bubbles, stains, and spots in combination with relative amounts and types of waviness, as well as other physical properties. In this system, a perfectly clear, transparent, flat specimen of mica is the visual standard of perfection. Increasing amounts of visual defects lower the visual quality, and a total of 13 levels of visual quality are covered by this standard. This method of classification, generally known as the Bengal India System, is purely qualitative and is entirely dependent on personal opinion and judgment.

The standards for visual quality classification that are covered in this classification are the best commercially available concept of the various qualities and their relative positions. Variations in the methods of using and applying these standards from those herein defined may be specified by the purchaser, or defined by agreement between the supplier and the purchaser.

Standard size classifications are defined, based upon available usable rectangular areas and the minimum dimensions of the rectangles that the pieces will yield. Precautions to be taken in making thickness measurements are also described.