152 Field Columbian Museum— Geology, Vol. III. 



OLIVENITE 

 TINTIC DISTRICT, UTAH 



PLATE LII 



Crystals of olivenite from this locality have been previously de- 

 scribed by Washington,* but a large suite of specimens received from 

 Maynard Bixby affords some new characters which seem worthy 

 of description. The olivenite in these specimens occurs both as well- 

 defined crystals and in the fibrous form known as wood-copper. For 

 the most part the crystals present the dark olive-green color char- 

 acteristic of olivenite, although there are some variations from this, 

 as will be noted. None of the crystals is highly modified, nor are they 

 of large size. For the most part they exhibit a prismatic habit and 

 occur encrusting cavities in a cupriferous gangue. The largest crystals 

 noted (Mus. No. M 9414) are represented by Fig. 1, PI. LII. These 

 crystals are scattered in radiated fashion over a siliceous matrix and 

 reach in some cases a length of 1 cm. They are usually attached 

 by the macropinacoid a (100). As shown in the figure, they are 

 simple in form, being made up of the unit prism m (no), the 

 macropinacoid a (100) and the brachydome d (025). This dome is a 

 form new to olivenite. Its determination was based on a good meas- 

 urement oi d a d' = 32 45'. A somewhat similar habit is ex- 

 hibited by the crystals shown in Fig. 2, PI. LII (Mus. No. M 9400), 

 except that the basal plane occurs here and the macropinacoid 

 is lacking. The dome and base are characterized by striations 

 || to a (100). These crystals are of dark, nearly black, color, about 

 1 mm. in length and occur thickly encrusting a somewhat porous 

 gangue. Another simple habit consists only of the unit prism and 

 basal plane, producing a tabular form. This is exhibited in Fig. 3, 

 PI. LII (Mus. No. M 9413). These crystals occur lining a cavity about 

 one inch in diameter. Sheaf-like crystals of azurite of a tabular habit 

 are implanted upon the olivenite. The olivenite crystals are of a light 

 olive-green color, with dull planes, and are usually attached by the 

 basal plane. The average length of these crystals, measured in 

 the direction of the macro-axis is 5 mm. A rather unusual habit for 

 olivenite is that represented in Fig. 4, PI. LII (Mus. No. M 9421). 

 These crystals are elongated in the direction of the brachy-axis. 

 The extension seems to be rather the result of growth of a number of 



♦Am. Jour. Sci., 1888, 3, 35, p. 298. 



