Ford and Tillotson — Orthoclase Twins. 149 



Art. XYIII. — On Orthoclase Twins of Unusual Habit y by 

 W. E. Foed and E. W. Tillotson, Jr. 



The orthoclase twins to be described in the following pages 

 were collected by Prof. L. V. Firsson during the summer of 

 1896, while engaged in work for the United States Geological 

 Survey in Montana, They occurred as phenocrysts in an altered 

 tinguaite porphyry which lay as an intruded sheet between 

 black shales near the head of West Armell Creek in the Judith 

 Mountains. The tinguaite sheet is described'" as measuring 

 " between ten or twelve feet in thickness with numerous 

 immense feldspar phenocrysts. some of them being four by two 

 by one inches across. " The groundmass of the rock is fine- 

 grained with a greenish gray color, the green tone being due to 

 the presence of fine crystals of segirite, while on the weathered 

 surfaces it is spotted with numerous pits stained yellow with 

 iron oxide. The writers desire to express to Frof. Firsson 

 their thanks for the opportunity to figure and describe these 

 crystals. 



The phenocrysts occur as well-developed crystals and, as has 

 been said, are at times of considerable size. They are opaque 

 and are frequently stained on the surface with iron oxide or 

 colored green with a thin coating of se^irite. When broken, 

 however, they present a glassy luster and fresh appearance. 

 The crystal faces were too rough to admit of measurements 

 other than those with the contact goniometer, but the forms 

 present were easily identified in this way and by their zonal 

 relations. They were all common forms comprising o (010), 

 c (001), m (110), z (130), n (021) and o (111). A few crystals in the 

 suite were untwinned and possessed a development as repre- 

 sented in figure 1, but for the most part the phenocrysts were 

 twinned according to the Baveno law in which n (021) be- 

 comes the twinning plane. They differ markedly in habit, how- 

 ever, from the common form of Baveno twins in that instead 

 of having the composition plane symmetrically placed in diag- 

 onal position through the square prism-like crystals, they are 

 rather in the nature of contact twins having the two individuals 

 more or less completely developed and grown together at right 

 angles to each other without much interpenetration. Figures 

 2 and 3 illustrate this peculiarity of development. In these 

 cases the division between the two individuals can be distinctly 

 traced, and is practically a single plane. In the crystal illus- 

 trated in figure 2, the individual drawn in normal position is 

 quite completely and symmetrically developed and the smaller 

 * Weed and Pirsson, U. S. G. S., Ann. Rep., 1896-7, iii, 524. 



Am. Jour. Sci.— Fourth Series, Vol. XXVI, No. 152.— August, 1908. 

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