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 a a 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.

*

*

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 eutectic 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.

b Teall, J. J. H., British petrography, 1888, p. 402.

e 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.

CHEMICAL COMPOSITION.

a Bygden, A., Über das quantitative Verhältniss zwischen Feldspat und Quarz in Schriftgraniten: Bull. Geol. Inst. Univ. Upsala, vol. 7, 1904, pp. 1-18.

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.

c Bureau of Mines samples.

d Geological Survey samples.

FegO3 included with Al2O3.

Sample A.-Fine-grained graphic granite (see Pl. I, B) from newly opened quarry of Maine Feldspar Co., east slope of Mount Ararat, Topsham, Maine.

Sample B.-Medium-grained graphic granite (see Pl. I, B) from Wm. Willis quarry, Cathance, Maine.

Sample C-Coarse-grained graphic granite (see Pl. I, B) from Golding Sons Co. quarry, Georgetown Peninsula, Maine.

Sample D.-Coarse graphic granite, Fisher's quarry, Topsham, Maine. Quartz layers about 1 inch across and feldspar layers about 4 inches across.

Sample E.-Moderately coarse graphic granite, Fisher's quarry, Topsham, Maine. Quartz layers about 0.05 inch across and feldspar layers about 0.15 inch across.

Sample F.-Fine grained graphic granite from Kinkle's feldspar quarry, Bedford, West Chester Co., N. Y. Quartz layers about 0.03 inch across and feldspar layers about 0.08 inch across.

Sample G.-Graphic granite from Andrews quarry, Portland, Conn. Quartz layers vary but average not more than 0.02 inch across. The feldspar layers average nor more than 0.05 inch across. Some areas of pure feldspar in the sample increase the the feldspar content shown by the analysis above what a graphic granite of this fineness should yield.

Study of the analyses A, B, C, D, and E, all of which are from the chief feldspar-producing district of Maine, shows no greater difference in chemical composition than would be expected in samples taken from different places in a single quarry. The maximum variation in potash content is 0.47 per cent, in soda 0.25, in total alkali only 0.53, in alumina 0.70, and in silica 0.64 per cent. The calculated mineral compositions indicates that the maximum variation in free silica content is only 3.4 per cent.

From the foregoing it appears that the statement of Vogt regarding the ratio of feldspar and quartz seems to hold for Maine graphic granites. A comparison of the calculated mineral compositions indicates that there is no relation between the variation in free quartz content and the size of quartz particles.

RELATION BETWEEN PYROMETRIC BEHAVIOR AND FELDSPAR

CONSTITUENTS.

The next problem was to determine the relation between the pyrometric behavior of the graphic granites and of their feldspar constituents. For this study, three graphic granites, A, B, and C, were selected which grade from a very fine to a very coarse grained granite. The quartz in all three samples was of the smoky variety and the feldspars are indicated by optical analyses to be microclines with small intergrowths of albite.

PREPARATION OF SAMPLES.

The three samples were photographed (Pl. I, B) and then each sample was divided into two equal parts; one part was pulverized as pegmatite and the other part carefully cleaned of all free quartz and pulverized as pure feldspar. In each case the pulverized material was ground dry to pass a 200-mesh sieve, mixed with a small amount of dextrine, 4492°-Bull. 92-16- -5

and molded in the plastic state into cones similar to standard pyrometric cones. These cones were tested as follows: (a) For the deformation temperatures and rates of deformation of the graphic granites as such; (b) for the deformation temperatures and rates of deformation of the pure feldspar components; (c) for the relation between the graphic granites and mixtures of their feldspar components and known proportions of pulverized quartz. The quartz used in this last test was quartz sand pulverized dry to pass a 200mesh sieve.

RESULTS OF TESTS.

The relative pyrometric behavior of the graphic granites and their feldspar components is shown in figure 13.

The diagram shows that at least in the case of these graphic granites of Maine the deformation temperature lies well beyond that

FIGURE 13.-Deformation of graphic granites and their feldspar components.

of their feldspar components. The first of the feldspars to commence deformation was C, followed within a few degrees by feldspars B and A. The feldspars all deform at about the same temperature as cone 8, feldspar C deforming at a temperature slightly lower and feldspars A and B at one slightly above cone 8. The rate of deformation of the three feldspars varies slightly, that of feldspar C, which begins to deform first, being the highest. Feldspars A and B both deform at the same rate as cone 8.

The three graphic granites of which these three feldspars formed a part do not deform until cone 9, and furthermore do not deform in the same order as the feldspars, but follow the order of the calculated free quartz content. Graphic granite A, which has a calculated free quartz content of 25.8 per cent, begins to deform noticeably ahead of the other two granites and completes deformation distinctly ahead of them. Graphic granites B and C, carrying

respectively 27.6 per cent and 27.4 per cent of free quartz, begin to deform at the same temperature, but C deforms more quickly than B and is completely deformed when B is not more than half deformed. The results of the analyses show that graphic granite C has the lowest alumina content and the highest soda content, both of which factors tend to favor rapid deformation in the pure feldspar. However, the small excess of free quartz in this granite more than counteracts the effect of the more fusible feldspar and causes the granite to deform at a higher temperature than graphic granite A, which contains slightly less soda and slightly more alumina but contains 1.6 per cent less free quartz.

Graphic granite B, which is the richest in free quartz and contains the highest percentage of alumina and the lowest percentage of soda, is the most refractory of the three samples, and its feldspar component is more refractory than that contained in either of the other two graphic granites.

Thus it is clear that a slight variation in composition may greatly change the temperature and rate of deformation of a graphic granite. It must be remembered, however, that the three graphic granites tested are so similar that in a commercial plant the difference in pyrometric behavior would hardly be noticed.

The main point brought out by this experiment is the fact that graphic granites high in potash and low in soda and containing no lime, of which the graphic granites of Maine are examples, deform at temperatures distinctly higher than do their feldspar components. Hence they are not deformation eutectic mixtures of feldspar and quartz. Whether these graphic granites are true eutectic mixtures, in that the ratio of feldspar to quartz represents the saturation point of each in the other so that both may solidify at the same time, is a question which this study was not designed to prove.

DETERMINATION OF QUARTZ CONTENT.

The determination of the quartz content in the graphic granites by synthesis resulted as follows:

Graphic granite A was very fine grained. A block of this material was crushed and then cleaned as completely as possible of free quartz. The fact that the quartz present was of the smoky variety made its detection less difficult than would otherwise be the case. The pure feldspar was pulverized to pass a 200-mesh sieve and a series of mixtures with pulverized quartz prepared containing 5 to 35 per cent quartz. The mixtures were molded into cones of standard dimensions. This series was placed on a fire-clay slab, together with a specimen of the original graphic granite A and also a specimen of the pure feldspar A.

The pure feldspar deformed with cone 8. The graphic granite A deformed slightly above cone 8 and simultaneously with the mixture