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Kaolin was added to a 1 to 9 mixture of quartz-feldspar in the percentages 0, 2, 4. 6, 8, and 10. The results of two burns are given in figure 11. The deformation
Cone 6 Cone 7 Cone 8
FIGURE 10.-Deformation of refined kaolin-feldspar cones.
took place with cones 8 and 9, the same as with kaolin and feldspar alone. With 8 per cent kaolin went the most rapid deformation, 10 per cent and 0 per cent kaolin
FIGURE 11.-Deformation of kaolin-feldspar-quartz cones.
came next, with a lesser rate, leaving 2 to 4 per cent as the "high" region and the slowest to deform.
Hence, in a general way, the feldspars with both quartz and kaolin impurities may be made to deform with and before pure feldspar, especially when in the region of 8 to 10 per cent kaolin, 9.2 to 9 quartz, and 82.8 to 81 per cent feldspar.
MIXTURES HIGH IN KAOLIN.
Dr. Simonis (Sprechsaal, 1907, vol. 2) gives the following table for the deformation of kaolin-feldspar mixtures:
Deformation points of kaolin-feldspar mixtures.
As is shown by figure 12, we partly checked this with our kaolin and feldspar, the slight variations being probably due as much to furnace irregularities as to anything else. Standard sized cones were made of the different mixtures and tested with standard cones in a small gas-fired muffle furnace.
In the figure the dotted lines show where our results disagree with those of Simonis. Notice the rise in deformation of 12 cones between the limits of 30 to 40 per cent kaolin.
GRAPHIC GRANITES AS A SOURCE OF FELDSPAR.
As the exposed deposits of pure feldspar in the United States are rapidly being worked out and the locating of new deposits of pure material is a matter of more or less uncertainty, it seems wise to seriously consider the available sources of feldspar which is not pure but contains no impurity that would make the feldspar unfit for ceramic uses. The most fruitful source of such investigation is the graphic granites or graphic pegmatites. The pure feldspar has been obtained from isolated lenses in these pegmatite dikes, and in mining it vast quantities of pegmatite rich in feldspar and containing little or no impurity, except quartz, were quarried. This was rejected as waste. When the lenses of pure material were becoming exhausted, the operator began to work over these waste heaps and also to quarry the walls adjoining the pure feldspar lenses, and he found there much material free from other impurity except quartz. This material was sorted, and the portion in which the quartz content did not exceed 10 or 15 per cent was accepted and mixed with the pure feldspar. No serious difficulty resulted from the replacement of 5 or 10 per cent of the feldspar by quartz, as this amount of variation in the ordinary pottery body might often occur through the use of material in which an excess of moisture existed.
However, each year the supply of available pure feldspar becomes less, and also the part of the graphic granites which is richest in feldspar is being used up, leaving a poorer assortment from which to select. Owing to the gradual hardening of the commercial grade of New England feldspars, which is not due to any change in the feldspar
* * * * * * * * * * * * * * * * * =
70 80 90
Kaolin, per cent 0 10 20 30 40 100 Feldspar, per cent 100 90 80 70 60 50 40 30 20 10 0 FIGURE 12.-Deformation curve of kaolin-feldspar series. After Simonis. Dotted lines show where results of Wilson's tests disagree with those of Simonis.
itself, but merely to the increased quartz content, many users have turned to other sources of supply, thus fortunately leaving the New England feldspars to those manufacturers who found their use essential to proper color in their wares. This is especially true of the Maine feldspars, which are recognized as being superior in color to any other American feldspar except the North Carolina feldspars, the cost of which is prohibitive in many industries.
PROPERTIES OF GRAPHIC GRANITES.
Graphic pegmatites, or graphic granites as they are generally termed, do not differ in essential constituents from ordinary pegmatite, except that the associate quartz is distinctly crystalline, as is shown in Plate I, B. The remarkable similarity of the quartz crystals to Chinese lettering has led to the application of the term "graphic granite" or "Schriftgranit," as it is called in German literature.
The value of the graphic granites for ceramic purposes naturally depends on the amount of impurity present and it is worthy of note that when the feldspar and quartz are intercrystallized as in this graphic form the amount of other minerals present is much less than in other forms of pegmatite or granite. As quartz is the chief associate mineral, its effect on the deformation point of the feldspar is of importance to the ceramist.
FELDSPAR-QUARTZ RATIO IN GRAPHIC GRANITES.
Vogt has offered the theory that in graphic granite the quartz and feldspar are present in eutectic proportions. He presents as evidence the following analyses of graphic granites and their recalculation into feldspar and free quartz contents.
Composition of graphic granites.
In conclusion he says:
When the potash feldspar is in excess, the proportions of feldspar and quartz in graphic granites are absolutely constant or only very slight variable * From this we conclude with absolute certainty that the graphic granite is a eutectic mixture and has a composition approaching 74 per cent feldspar and 26 per cent quartz.
Teall concludes that the graphic granite represents the cutectic between feldspar and quartz.
Johnson concludes that the feldspar-quartz content in graphic granites has a definite ratio, dependent upon the type of feldspar present. When the feldspar is orthoclase the molecular ratio of feldspar to quartz is 2:3. When the feldspar present is plagioclase the molecular ratio is 1:2. When the feldspar present is albite the molecular ratio is 1:3.
a Vogt, J. H. L., Die Silikatschmelz-lösungen, 1904, pp. 117–128. Teall, J. J. H., British petrography, 1888, p. 402.
Johnson, H. E., Geologiska föreningens Förhandlinger, Bd. 27, 1905, p. 119.
Bygden concluded from a study of many analyses that the type of feldspar present bears no definite relation to the feldspar-quartz ratio. He believes that a definite feldspar-quartz ratio does exist in most graphic granites, but thinks it is not so simple as Vogt and Johnson imagine.
Bastin, after considering the data and opinions of other investigators, compiled a table of mineral compositions of graphic granite in which he included not only all available foreign data but also the compositions of three Maine graphic granites. From a study of these he finds that even among graphic granites whose feldspars are almost identical in composition, there are considerable variations in the feldspar-quartz ratio. Bastin's table of graphic granite compositions is included as analyses D to G in the table following.
VARIATION IN GRANITES THROUGHOUT A GIVEN DISTRICT.
The subject of chief interest to users of feldspar is the possible variation of graphic granites throughout a given district, both as regards ratio of feldspar to quartz and also as regards the chemical composition and physical behavior of the feldspar component of the graphic granites.
In the following table samples A, B, and C were collected by A. S. Watts, of the Bureau of Mines, and were analyzed by A. C. Fieldner of the bureau. Samples D to G were collected by the United States Geological Survey and were analyzed by George Steiger in the laboratory of the Geological Survey.
Composition of New England graphic granites.
d Geological Survey samples. Fe2O3 included with Al2O3.
Per cent. Per nt.
Trace. .do. None.
CALCULATED MINERAL COMPOSITION.
er cent. Per
aBygden, A., Über das quantitative Verhältniss zwischen Feldspat und Quarz in Schriftgraniten: Bull. Geol. Inst. Univ. Upsala, vol. 7, 1904, pp. 1-18.
b Bastin, E. S., Geology of the pegmatites and associated rocks of Maine, including quartz, feldspar, mica, and gem deposits: U. S. Geol. Survey Bull. 445, 1911, pp. 39-42, 124-125.
e Bureau of Mines samples.