The ordinary conception would lead to the assumption that No. 3 is the most easily fusible of the three instead of being the least fusible, as it really is.

EFFECT OF COMPOSITION ON PROPERTIES OF FELDSPAR.

DEFORMATION TESTS OF POTASH FELDSPAR-SODA FELDSPAR MIXTURES.

In order to determine the most fusible mixture of potash and soda feldspars, a series of mixtures of these two feldspars was prepared and deformation studies made.

The feldspars used in preparing the series of mixtures had the following composition:

Neither of the above feldspars is a pure type, each of them being a mixed feldspar that was high in the particular feldspar sought. Unfortunately, in all the large feldspar deposits the feldspar contains less than the theoretical amount of alkali, although as a rule the ratio of AO, to SiO, closely approaches the theoretical if the free quartz in the pegmatite is removed. This shortage of alkali is doubtless due to slight alteration of the feldspar, but no information is available pointing to the formation of products other than kaolin by the weathering of feldspar.

To subdivide an ordinary feldspar into its possible mineral components by a process of calculation based upon its alkali content results in confusion, as an excess of Al,O, and SiO2 remains after all possible minerals have been deducted. The absence of sufficient

combined water makes it impossible to calculate the excess ALO, into kaolin, although the Al,O, must be in other than the free state because none dissolves in hydrochloric acid.

As the feldspars used in this investigation must be divided into microcline and albite content, the ratio of K2O to Na2O has been chosen as the basis of division after that amount has been deducted which can be calculated as kaolin, the combined water being used as the basis of this latter calculation.

The result of this subdivision is as follows:

Composition of feldspars on basis of microcline, albite, and kaolin content.

The mixtures of these two feldspars as used in the investigation

The order and rate of deformation of these potash feldspar-soda feldspar mixtures, as compared with similar properties of standard pyrometric cones Nos. 6, 7, 8, 9, and 10, are shown in figure 1. The percentage composition of the feldspar mixtures are shown at the top of the diagram, and the time interval and temperature increase as indicated by deformation of the standard cones are shown at the left.

DISCUSSION OF RESULTS.

The data in figure 1 are remarkably sharply defined as regards the end numbers of this series, but unfortunately the deformation ranges of the three softest members are so nearly the same that a number of checks must be made before the eutectic can be established.

From this preliminary test it seems that a mixture of approximately 69 per cent albite and 29 per cent microcline begins to deform at a lower

FIGURE 1.-Deformation of potash feldspar-soda feldspar cones.

temperature and completes the deformation process earlier than any other member of the series tested. The time required for the deformation of a cone made from this mixture was only half that required for a similar deformation of standard cone 6, the cone starting to deform at approximately the same time as cone 6 and being completely deformed when cone 6 had only half deformed.

Mixtures ranging from one that was 77.5 per cent albite and 21 per cent microcline to one that was 44.5 per cent albite and 53 per cent microcline completely deformed before cone 6 had completed deformation. The albite used in this study deformed at about the same rate as the eutectic mixture, but proved to be nearly one cone harder. The rate of deformation of the mixtures containing a higher percentage of microcline than the eutectic mixture decreases as the microcline content increases, the potash-feldspar end of the series having a time range of deformation equal to twice that of the albite.

Thus it is shown that a high soda content does not indicate a low deformation temperature. Also the approximate proportion of soda feldspar can not be determined except by a careful observation of the rate of deformation.

DEFORMATION TESTS OF FELDSPAR-QUARTZ MIXTURES.

One of the most confusing questions which confronts the user of feldspar is the extent to which the addition of quartz or free silica affects the pyrometric behavior of feldspar. In nature quartz is almost invariably associated with feldspar, and in most cases a complete separation of the two minerals would be very expensive if not impossible. Any feldspar purchased in the pulverized state is, therefore, very likely to contain a certain amount of free quartz unless special arrangement was made to have all quartz removed from the crude feldspar. In order that a clearer understanding may be had of the effect of quartz on the different feldspars as regards deformation behavior, a study was made of potash feldspar-quartz mixtures and of soda feldspar-quartz mixtures.

The feldspars used were the same as those used in the potash feldspar-soda feldspar mixtures. These, as heretofore explained, are not pure minerals, but are as nearly pure as are obtainable in commercial quantities.

EXPERIMENTS WITH POTASH FELDSPAR-QUARTZ MIXTURES.

The composition of the potash feldspar and the quartz used was as follows:

Composition of potash feldspar and of quartz used.

20-MINUTE INTERVALS

The materials were ground to pass a 200-mesh screen and mixed in the proper proportions dry; the mixture was then made plastic by a small addition of dextrine solution. The various mixtures were molded into cones similar in shape and size to the standard pyrometric cones of commerce. The results of the tests are shown in figure 2. The proportions of feldspar and quartz in the various

FIGURE 2.-Deformation of potash feldspar-quartz cones.

mixtures are recorded across the top of the diagram, and the intervals of time and of temperature as indicated by the deformation of the standard cones are shown along the left margin.

According to the results given in figure 2 the mixture of 90 per cent potash feldspar and 10 per cent quartz began to deform and completed deformation before any other mixture, and is therefore the deformation eutectic of this feldspar and quartz. The intervals between the deformation of this mixture and those of the mixtures