Thermal behavior of muscovite sheet mica

Abstract

The three sp ect ral types of muscovite shee t mica, i.e. , very pink ruby, ligh t gree n, a nd dark green, w ere subj ected to heat tr ea tm ents a t tempera t ures up to 600°C. The c ha nges in the apparen t optic a xial a ngle and in the absorp tion spectra (0.3 to 15 JJ. ) were st udied along with color.The d iff erentiation of muscovi te sheet ac cording t o these spectral typ es extends to t he beha vior of apparen t op tic axial a ngle and to certain regions of t he spectrum under hea t treatmen t. The pink associated a bsorp tion region (0.47 t o 0.6 JJ. ) can be enhan ced or blea ched away by appropriate t hermal trea t men t, althoug h t he assoc ia ted infrared multiplet at 3 to 3.5 JJ. is lit tle a ffected . The abso rp tion ba nd at 12 JJ. increases in intensity wi t h te mperat ure o f t reatm en t. It is s uspected t hat t he 0.47 to 0.6 JJ. a bsorp t ion is t he r esult of color cen tel's.It has been shown th at m easurable differ ences exist in appar en t op tic a xial angle and absorp tion sp ectrum , as well as in color, for muscovi te sheet micas. These diff er ences indica te that there must b e basic ch emical and structural variations. They fur th er provide a quan ti tative, t hou gh complex, categoriza tion of the material [1). 1The present p ap er r eports the effect of h eat tr eatm en t on color, apparent optic axial angle, and absorp tion sp ec trum for sever al of th e r eprcsen Lative categories of the m aterial so e tablish ed . The tr eatm en ts wer e a t temper atures of 600°C and less, usually consider ed to b e below th e decomposition poin t . Experimental ProceduresThe details of m easurem ent of appar en t optic axial angle, absorption sp ectrum, and color ar e as b efor e [1,2) .Comparisons of sp ectral variations are again made in terms of the r esonance absorption coefficient (below b ackground), cx x(R ), defin ed with r espec t to an a,ppropriately pl aced base lin e as (1) wher e Til. is the transmission at t he r esonance band center , Tb is the value at the base line for t he same wavelength, and t is t he s pecimen thickn ess. Color was determined by compa rison with the previously selec ted standard samples. SpecimensThl'Oe categories of mu covite sheet wer e chosen . These wer e very pink ruby, light gr een , and dark 1 Figures in brackets indicate tb e literat ure references a t tho end of tb is paper. gr een , which represe n t the three end-typ es as d efin ed in th e earlier work.Crystal sh eets wer e all of high qu ali ty, V-I to V-4 [3), with origins in India, Brazil, T angan yika, and th e Uni ted States.As appropriate combin a tion of visual color determ in ation and r estriction to magni Lud e of apparen t optic axial angle can as ure selection of th ese endtyp es, su ch procedure was used for sample choice. Specimens were fir t selected according to visual color. They wer e t hen picked for m agni tude of appal'cn t op L ic axial angle. The very pink ruby specimel1:6 were 1'c trictecl Lo an0 1es between 68° and 72. 5°. Ligh t gr eens wer e restricted t...

Research on crystal growth and characterization at the National Bureau of Standards :

“…There have recently been published some additional results from the now concluded studies of muscovite mica (Ruthberg, 1963) . **…”