. Bulletin - United States National Museum. Science. THE MINERALS OF IDAHO 375 scent marcasite from the Garfield tunnel. This was analyzed with the following results: Analysis and ratios of clay from Garfield tunnel (E. V. Shannon, analyst) Per cent Ratios. Silica (Si02) Alumina (AI2O3) Ferric iron (Fe203> -. Lime (CaO) Magnesia (MgO) Potash (K20).. Soda (Na20) Water (H2O) above 110° C Water (H20) below 110° C Total The formula derived from this analysis is— 3(H,K) which is indeed like the composition which might be obtained from a mixture of muscovite and k
. Bulletin - United States National Museum. Science. THE MINERALS OF IDAHO 375 scent marcasite from the Garfield tunnel. This was analyzed with the following results: Analysis and ratios of clay from Garfield tunnel (E. V. Shannon, analyst) Per cent Ratios. Silica (Si02) Alumina (AI2O3) Ferric iron (Fe203> -. Lime (CaO) Magnesia (MgO) Potash (K20).. Soda (Na20) Water (H2O) above 110° C Water (H20) below 110° C Total The formula derived from this analysis is— 3(H,K) which is indeed like the composition which might be obtained from a mixture of muscovite and kaolinite. It is also similar to the composition obtained for the sericite from Boise County except in that the alumina is low and there is one molecule of water which is lost at 110° C. Optically the material is nearer leverrierite with the indices of refraction approximately a = , 7= , bire- fringence It is so exceedingly fine grained that it is not possible to say definitely that there is no amorphous material mixed with the minute crystalline flakes but the material all appears of the same refractive index and birefringence which is lower than for either muscovite or kaolinite. The evidence seems to be in favor of the conclusion that these clays are not so simple as just mixtures of kaolinite and sericite. Many of them may be pure sericite, even when low in potash, while others may be more properly classifiable as leverrierite, a mineral of similar properties whose exact relation to the muscovite and kaolinite groups is not yet known. These clays are further discussed under the headings leverrierite and kaolinite below. SHOSHONE COUNTY In Shoshone County extensive formations of quartzitic or slaty rocks are characterized by the more or less abundant occurrence of sericite throughout. The Burke formation is characteristically a slaty sericitic quartzite and the Wallace formation is, in places, practically a sericite slate or schist. The occurrence of thi
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