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2. W.C. Roberts, G.R. Rapp, Jr. and J. Weber,Encyclopedia of Minerals, Van Nostrand, New York (1974).
3. L.B. Berry and B. Mason,Mineralogy, Freeman, San Francisco (1959).
4. K. Frye,Encyclopedia of Mineralogy, Hutchinson Ross, Stroudsburg, Penn. (1981).
5. W.D. Deer, R.A. Howie and J. Zussman,Rock Forming Minerals, Vol. 1, Wiley, New York (1962).