GEOPHYSICAL LABORATORY. 169 



(23) On aventurine feldspar. Olaf Andersen. Am. J. Sci. (4), 40, 351-398 (1915). 



In this investigation a number of varieties of aventurine feldspars were 

 examined. Orientation angles of the reflecting lamellae were measured, chiefly 

 with the goniometer, and the properties of the lamellae were determined under 

 the microscope. Brief discussions of the optical problems are included in the 

 record of these examinations. 



The reflecting lamellae are always oriented after simple crystal forms, of 

 which (112), (112), (150), and (150) occurred as planes of orientation in all 

 varieties, the first two causing aventurization on (001), the last two on (010). 

 The forms (001), (010), (110), and (UO) also frequently contain reflecting 

 lamellae. Exceptionally (021) and (113) are planes of orientation. The 

 orientation of the edges of the lamellae was evidently regular, but simple 

 crystallographic relations could not, in general, be found. 



The reflecting lamellae were determined as hematite. They vary widely 

 from one variety to another as to shape and size, showing hexagonal, eight, 

 or ten-sided, rhomb-shaped, strip-shaped, or irregular outlines. The largest 

 measured 3.5 mm. in one direction, the smallest were of submicroscopic size. 

 The absorption colors were those characteristic of hematite. It was shown 

 that the colors in reflected light are interference colors of thin films. By 

 means of these colors the thickness of the transparent lamellae could be 

 approximately determined. It was found to vary between 50 mm and 500 ijlix. 

 The lamellae were shown to possess no appreciable pleochroism. The apparent 

 pleochroism observed in lamellae forming large angles mth the section was found 

 to be due to the effect of polarization by reflection and refraction at the surface 

 of the lamellae. The appearance of interference spectra in these lamellae is due 

 to the action of the lamellae as polarizers or analyzers for the wedge-shaped parts 

 of the feldspar that lie above or below them in the sections. 



Thermal experiments with one of the varieties showed that the hematite 

 lamellae persist up to about 1235°. At this temperature they disappeared, 

 presumably by melting together with a small part of the surrounding feldspar 

 to thin, invisible glass films. The feldspar remained otherwise unchanged 

 (crystallized). By heating at lower temperatures some of the lamellae (the 

 originally opaque ones) reappeared in the same places and with the same out- 

 lines as before. By a long heating at temperatures around 1050° (of cleavage 

 pieces not previously heated) the opaque lamellae generally became trans- 

 parent and the others became a little lighter in color. 



The origin of the hematite lamellae was explained as due to unmixing of an 

 originally homogeneous feldspar which contained iron oxides in solid solution. 

 Thin lamellae of hematite then separated along certain structural planes of 

 the feldspar. 



In the concluding section all the specimens examined are described and the 

 measurements tabulated. 



(24) Aventurine Feldspate. Olaf Andersen. Z. Kryst. (in press). 



A German translation of "On aventurine feldspar" (Am. J. Sci. (4), 40, 351- 

 398, 1915). Reviewed under No. 23 above. 



(25) Der optische Charakter der schwachen, mit starken Objektiven zwischen gekreuzten 



Nicols beobachten Interferenzfigur. Fred. E. Wright. Z. Kryst., 55, 115-122 

 (1915). 



A German translation of "The optical character of the faint interference 

 figure observed in high-power objectives between crossed nicols" (J. Wash. 

 Acad. Sci., 4, 301-309, 1914). Reviewed in Year Book No. 13, p. 147. 



