Parallel extinction

The elongated sample is next rotated parallel to an eyepiece crosshair, and one notes if the sample goes to extinction if it does, it has parallel extinction (the vibra-... 

The distinction between the vibration directions of higher and lower refractive indices can always be made in this way for crystals having inclined extinction no less than for those with parallel extinction. When refractive indices are measured by the methods already given, the use of the quartz wedge is hardly necessary (unless for confirmation of conclusions already reached) but in other circumstances (for instance, when crystals are being examined in their mother liquor), quartz wedge observations are useful clues to optical character. 

Monoclinic minerals have an inclined angle of extinction. Orthorhombic minerals have parallel extinction. Chrysotile can be monoclinic or orthorhombic depending upon whether it is the ortho or clino variety. The b axis of the clino variety is so close to 90° (93°), (25) that the fibers will appear to have parallel extinction unless this is carefully measured. Anthophyllite, an orthorhombic mineral, has parallel extinction that is, the angle of extinction is zero degrees. All amphibole asbestos minerals except anthophyllite are monoclinic. Although wollastonite is a triclinic mineral, its extinction is parallel, or nearly parallel. The angle of extinction of some asbestos minerals is shown in table I (26, 27). 

If a mineral is known to be either anthophyllite or tremolite by dispersion staining tests, the angle of extinction can then be used to distinguish between the two. Caution It is possible for a mineral which usually has inclined extinction to have a few fibers with parallel or close to parallel extinction, depending upon orientation. These fibers can be rolled into a position of maximum extinction. (See section on rolling fibers.)... 

Natural anhydrite is orthorhombic with good cleavage. It is almost always seen as blocky crystals with 90° comers. The refractive index is 1.570 to 1.614, which is much higher than gypsum, so it stands out when we mount the specimen in 1.528 RI to make the gypsum disappear (Fig. 23). The birefringence is 0.044 RI, which means that particles as small as 10 p.m will show some color. As a matter of fact anhydrite is very colorful in the sizes we ordinarily encounter and is easy to recognize. It has parallel extinction that helps to confirm our identification (Fig. 15). 

It shows parallel extinction and is length-fast. No difference in optical characters was found between the 2 types, and zonal structure is not observed microscopically. The x-ray powder diffraction pattern agrees with that of the Patagonia clinoptilolite (12). The Nakanosawa clinoptilolite was unchanged up to 700°C, thus fitting Mumptons criterion (15). It was not destroyed by hot 6N HC1 in 2 hours. 

The residue from the above is a hard, very tough, opaque, leatherlike mass. It melts (becomes transparent) at 58°-63°C, but at this temperature it is too stiff to flow and shows considerable resistance to deformation. At slightly higher temperatures, it can be drawn out into thin strips or filaments, which can be stretched and cold drawn (see Note b). The product then shows fiber orientation and also exhibits parallel extinction between crossed Nicols. The cold-drawn material is exceedingly strong, tough, and pliable. 

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