Day and Shepherd — Lime-Silica Series of Minerals. 297 



1*615 7 = 1*636 ; its birefringence is strong and considerably 

 higher than in wollastonite. 



Silicon dioxide. — In nature this oxide occurs in at least 

 two modifications, quartz and tridymite, both of which were 

 produced artificially in this laboratory. As a result of the 

 experiments, the fact is well established that quartz is the 

 stable form below about 800° and tridymite over the range 

 800° to the melting temperature 1600° ; that, on heating, the 

 inversion of quartz into tridymite is so extremely sluggish 

 that quartz crystals may be heated 700° or 800° above the 

 inversion point without changing. Quartz glass, however, can 

 be made to crystallize to tridymite as low as 1000°. 



The microscopic examination of the different artificial prep- 

 arations of tridymite and quartz proved satisfactorily their 

 identity with the natural minerals. By using the method of 

 refractive liquids it was found possible to discriminate 

 between quartz, tridymite and amorphous quartz with ease, 

 even though many of the preparations were cryptocrystalline 

 and scarcely determinable by other ordinary methods. The 

 refractive index of amorphous quartz, obtained by precipita- 

 ting silica from solution, was measured by the immersion 

 method at l*459db*003; later the same constant was determined 

 more accurately on a polished face of quartz glass* on an 

 Abbe total refractometer in sodium light with the result, 1*460. 



(a) Quartz. — The best crystals of quartz were obtained as 

 a byproduct from a mixture of magnesium-ammonium chlor- 

 ide, sodium metasilicate and water heated for 3 days in a steel 

 bomb at 400-450°. This mixture was used by Dr. E. T. Allen 

 of this laboratory to synthesize one of the polymorphic forms 

 of magnesium metasilicate and to procure measurable crystals 

 of the same. The quartz crystals thus procured were color- 

 less, water clear, doubly terminated and well developed crys- 

 tallographically. The larger crystals attained a maximum 

 length of 2 mm , but were usually coated with a thin film of 

 foreign matter and were less suited to goniometric measure- 

 ment than smaller ones. The crystals are often barrel-shaped 

 with short rhombohedral terminal faces which pass by oscilla- 

 tory development into steeper rhombohedrons and finally into 

 the prism which shows the characteristic striae of the mineral 

 quartz. In a few of the crystals, the positive rhombohedron 

 only was developed and the crystals were terminated by its 

 three faces above. Several crystals were measured on the 

 two-circled goniometer and the forms (1010) (1020) (1011) 

 observed. Other forms were noted but gave indistinct and 

 multiple reflection signals and are not listed in consequence. 



* Obtained by fusing quartz in the electric furnace under pressure. 



Am. Jour. Sci.— Fourth Series, Vol. XXII, No. 130.— October, 1906. 

 21 



