101 Hypersthene Crystals

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

BASALTS.

INTERSTITIAL BASALTS.

HYPERSTHENE, APATITE, AND PSEUDOBROOKITE CRYSTALS IN BASALT.

HYPERSTHENE CRYSTALS.a

The hypersthene is in the form of minute tabular crystals, 0.5 to 0.75 millimeter in length, 0.3 to 0.5 millimeter in breadth, and about 0.1 millimeter in thickness. They are always attached to the walls of the cavity by an edge in such manner that double terminations never occur. The faces are bright and sparkling except a, 100, which is sometimes striated parallel to the vertical direction; but on account of their minute size the reflections are dim, and the readings, therefore, subject to considerable variations.

aThe description here given is the work of Dr. C. Palache.

The observed forms (position of Groth, a: b: c=1.0308: 1: 0.5885) were as follows:

a, 100 n, 210 i, 121
b, 010 w, 310 u, 322
m, 110 k, 102 —-

The most frequent combination of these forms is shown in fig. I of Pl. XIV (p. 76), which is an exact reproduction of the figure given by Schmidta for the hypersthene from Málnás, Hungary.b

aZeit. für Kryst., vol. X, 1885, p. 210.bThis figure is given by Dana, p. 349, fig. 2, but with incorrect lettering. With the axes there adopted, the letters a and b on the two pinacoids should be interchanged.

k, 102, also found on the Málnás crystals, was found only on a single crystal shown in fig. J of Pl. XIV, and the same was true of n, 210, a single narrow face not shown in the drawing. w, 310 was observed on several crystals and measured on two. This form was first observed by Fouqué on hypersthene from Santorin,c but he did not indicate it as new and gave no measurements. It has therefore been rejected in most of the recent lists of hypersthene forms, but appears to have been confirmed by the present measurements.

cBull. Soc. min. France, Vol. I, 1878, p. 47.
  Measured. Calculated. Number
of
readings
? p ? p
  o o o o  
m, 110 44 12 90 00 44 07 90 00 5
n, 210 62 46 90 00 62 44 90 00 1
w, 310 71 24 90 00 71 02 90 00 5
k, 102 90 00 15 55 90 00 15 56 2
i, 121 25 55 52 34 25 52 52 36 3
u, 322 55 27 46 06 55 30 46 06 2

 

 

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