420 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 59. 



PHYSICAL PROPERTIES. 



Creedite is ordinarily colorless, but many of the radiated masses 

 show broad bands of a beautiful, delicate purple color. The luster 

 is vitreous. The cleavage is pinacoidal, parallel to the 100 face. 

 The mineral is brittle and breaks with a concnoidal fracture. The 

 hardness is 4. The specific gravity, determined by the pyenometer 

 method, is 2.713. (Larsen and Wells give the specific gravity as 

 2.730.) The common mode of aggregation is in radiated masses. 



OPTICAL PROPERTIES. 



The optical properties given below arc those determined by Larsen. 

 The indices of refraction of the new material were measured and 

 found to agree with those given below. The mineral is optically 

 negative. The optic axial angle as measured is: 



2V Li = 64° 30' ±10'; 2V Na = 64° 22' ±10'; 2V Tl = 64° 20' ±10'. 



The dispersion was perceptible only on one axis. Extinction 

 angle 42° 30' ± 30'. Optical orientation, Y = b. 



The indices of refraction are: 



a=1.461±001 

 l i=1A78±00\ 

 7=1.485±001 



The axial angle calculated from these indices is 65°, agreeing well 

 with the observed ones. 



CRYSTALLOGRAPHY 1 . 



There are two main habits of crystals, those of the first type 

 occurring in the fluorite, those of the second in the halloysite. The 

 crystals of type 1 are prismatic with an equal development of the 

 front and rear pyramids. The base on these crystals is sometimes 

 absent but generally well developed and sometimes sufficiently large 

 to reduce the pyramids to narrow faces. The crystals of the second 

 type are also prismatic, are doubly terminated, and with a very 

 prominent development of the front pyramid. The rear pyramid 

 and base are reduced to almost minute size. The figures show some 

 of the various types drawn so as to bring the plane of the clino- 

 pinacoid to the front. 



In the calculation of the elements only the faces of the unit pyra- 

 mid could be used. The orthodome zone, including the base, was 

 in all cases when present considerably etched. The crystals from 

 the clay were brilliant, but the faces were invariably curved and 

 yielded a number of signals. The following are the angles for the 

 unit pyramid as measured and designated as excellent. 



