14 



PEOCEEDINGS OF THE NATIOITAL MUSEUM 



VOL. 71 



Another single specimen (No. 84549) is labeled " garnet on limo- 

 nite " and consists of a heavy iron-stained mass mainly made up of 

 blackish garnets of dodecahedral form narrowly truncated by the 

 octahedron. Under the microscope the garnet is colorless, but con- 

 tains opaque brownish material. Its index is, for the most part, 

 1.736, but varies to 1.745, It is probably grossularite. 



DIOPSIDE 



Diopside is a common mineral and occurs in many specimens, in 

 some as almost ideally perfect twinned prisms, in others as both 

 small twinned prisms and simple crystals of small size but perfect 

 form. Moreover, this mineral occurs in massive form, making up a 

 solid green diopside rock, and also mixed with more or less garnet 

 in a hornfels which is the base of most of the fine specimens of 

 vesuvianite and grossularite. 



Most noteworthy are the twinned diopside crystals which attain a 

 size of 8 by 10 mm. or more. These are gray-green or olive gray- 

 green in color and rest upon the walls of cavities in massive diopside 

 rock. Practically all of the crystals are contact twins on (100) and 

 the larger ones are rather highly modified. The interstices be- 

 tween the diopside crystals may contain later epidote, andradite 

 garnet, or calcite. A crystal typical of the best and larger twins is 

 shown in Figure 4. This gave the following measurements : 



Measurements of diopside, Figure 4 



Form 



No. Letter 



Symbol 



Odt. MUler 







ooO 

 Ooo 



01 

 -10 



+ 12 

 -12 



001 

 100 

 010 

 Oil 

 101 

 110 

 121 

 121 



Quality description 



Medium 89 14 



Poor, narrow.. 89 14 



Excellent 00 



do 25 39 



do 90 00 



do 43 35 



Good 35 18 



Excellent 13 25 



Measured 



15 38 



90 00 



90 00 



33 24 



15 55 



90 00 



55 00 



50 36 



Calculated 



90 00 



90 00 



00 



25 43 



90 00 



43 33 



35 36 



13 12 



15 51 



90 00 



90 00 



33 00 



15 27 



90 00 



55 24 



50 27 



This figure is drawn with the position of the clinopinacoid, h (010) 

 in front. When one of these larger twinned crystals is crushed and 

 examined under the microscope as a powder of 80 mesh or under it 

 is found to be pure and practically colorless. The indices are subject 

 to a slight variation from grain to grain but the material is fairly 

 homogeneous. The optical properties, mean, as measured are: Bi- 

 axial negative with 2V medium, dispersion, r < v, pronounced. The 

 average indices of refraction are, a =1.676, /3= 1.683, y = 1.702. The 

 extinction, Z A c, is 36° average of a number of measurements on 



