Page images
PDF
EPUB

The table discloses the fact that the ratio of potash feldspar to soda feldspar in graphic granite is generally greater than 2:1 and often reaches 3:1. In those samples in which soda feldspar is the prevailing type of feldspar the potash feldspar is present in very small amount only. The graphic granites 15, 16, 17, and 18 belong to a class entirely distinct from the others. A fact worthy of note is the small variation in norm content of potash feldspar in Nos. 1 to 14, inclusive. With the exception of Nos. 2 and 13, the potash feldspar content is between 47.4 per cent and 55.2 per cent of the graphic granite, whereas the soda feldspar in the same series varies from 10.6 per cent to 29.2 per cent. In the first 14 samples-that is, the graphic granites high in potash feldspar-only two contain more than 27.6 per cent free quartz and only two contain less than 23.2 per cent. Thus 70 per cent of the graphic granites studied contain 23.2 to 27.6 per cent of free quartz, a variation of 4.4 per cent.

PROSPECTING AND SAMPLING.

Too much emphasis can not be placed upon the importance of thoroughness in prospecting for feldspar and in sampling the deposit when located. Practically all the failures in feldspar quarrying are due to improper prospecting and sampling, which has led to an overestimation of the extent of the deposit and of the quality of material obtainable. In many instances the analysis of hand specimens taken in the course of ordinary prospecting have been made the basis of large investment and expenditures where the material of the grade sampled is so limited as to hardly justify a passing glance.

In prospecting for feldspar the geology and topography of the district must be carefully considered. If the country rock is more easily eroded than granite or pegmatite the dikes or sills are exposed above the surrounding country. If the country rock is more resistant to erosion than granite or pegmatite, the existence of the dikes or sills is indicated by depressions. Throughout the New England and North Appalachian States the former case prevails and in many places pegmatite rich in feldspar is exposed along the ridges or along the steep slope of the hills.

When such an exposure is found or a deposit of pegmatite is in any way discovered, the prospector should first ascertain the general nature of the walling rocks. From a hasty survey the existence of folding or faulting can be determined and also the deposit classified as dike or sill. An exposure may be either across the dike or along its strike and what may seem to be the face of a broad lens may be in reality only a thin sheet. With the formation identified as dike or sill the prospector should proceed to determine the strike and dip as well as the general dimensions of the deposit. To do this the

overburden should be removed from the exposure until the full width of the deposit is bare. With the data thus obtained the apparent strike should be projected 50 feet and the deposit uncovered at that point. Sometimes this projected continuation of the dike will prove erroneous, hence it is advisable where the overburden is heavy to locate the dike first by drilling. Prospecting should be continued at intervals of not more than 50 feet in both directions from the original exposure, and if the deposit seems to vary greatly in dimensions the intervals should be reduced to 25 feet.

The location of the dike by drilling is only of value as a labor saver in determining where the cuts shall be made. The reason for this is obvious. The drill sample can merely indicate that the formation is of a granitic nature and shows little as regards size of grain or the extent to which impurities exist.

After locating the limits of the deposit therefore open cuts or at least a pit should be made in order to expose a reasonable area of the surface. These exposures should not be farther apart than 50 feet. The cutting of a trench across the entire deposit as compared with sinking a pit through the overburden has been proven by experience to be little if any more expensive, as the trenching can be done with plow and scraper, whereas in digging a pit most of the work is necessarily hand labor.

Which surface is exposed by this method depends on the nature of the deposit, if it is a dike the surface exposed represents the crosssectional face of the deposit; if a sill the surface exposed is one of the walls and does not in any way represent an average of the deposit. The mode of procedure therefore depends upon the nature of the deposit.

SAMPLING A DIKE.

To sample a dike from the trenches cut, the most satisfactory method is as follows: Lay off the dike from wall to wall in 5-foot centers, beginning at a point 1 foot from the foot wall and placing the last center not more than 3 feet from the hanging wall. At each of these centers drill a row of three holes, 3 feet deep and 2 feet apart, on a 45° slope toward the hanging wall, the rows being at right angles to the trench wall. Charge each hole with one stick of "60 per cent" or one and one-half sticks of "40 per cent" dynamite, and tamp well with soft clay after attaching the detonator or exploder wire. Fire by whatever method is most convenient, but fire the rows in regular order, beginning with the one next the hanging wall. Much better results will be obtained if all the holes of a row are fired simultaneously. Each row should be carefully sampled and all loose débris cleaned away before the next round of shots is fired. As soon as the shots are fired, the loosened material should be removed and broken into pieces of approximately 3 inches diameter or less.

This material should include all that is loosened by the shot, but judgment should be used in handling it-for instance, a block of mica which was imbedded in the mass, but was entirely independent of the remaining material, should not be crushed and mixed with the other material loosened by the shots.

A careful record of the surfaces exposed should be made and especial emphasis should be placed on the gradation from the surface inward as regards size of grain and presence of associated minerals.

The material loosened by a row of shots should all be piled on a sampling cloth or wooden platform and thoroughly mixed, quartered, and two opposite quarters discarded. After mixing the remaining material thoroughly, it should be quartered, and two opposite quarters discarded. What remains on the cloth should be broken into pieces not to exceed 2 inches maximum diameter and the whole thoroughly mixed together. By repeated quartering this mass should be reduced to a sample of about 5 pounds.

In quartering especial care should be taken that the quarters discarded are entirely removed, because if some fine material is left on the sampling cloth each time, as is sometimes done by careless samplers, the resultant 5 pounds will contain an excessive amount of this fine material, which may have a different composition from the coarser material.

If the faces exposed by the shots indicate that the dike material improves rapidly with depth it may be advisable not to sample the rock next the surface, but to obtain the sample from the bottom of the holes. This can sometimes be done by striking with a heavy sledge the faces exposed in the bottom of the blast hole. If this is not effective a single hole may be bored in the bottom of the blast hole. This drill hole should slope 45° toward the foot wall, thus cutting under the face exposed by the first shot with the minimum depth of drill hole. A single hole 2 feet deep charged with one stick of "60 per cent" dynamite will generally move enough pegmatite to afford a good sample and this sample should be all removed and carefully sampled as explained above. After all the centers in all the trenches have been sampled in this manner, the depth of the dike is still to be determined. As this involves considerable expense for sinking shafts or cutting a tunnel, it may be advisable to first make or have made fire tests of the samples already taken. If the results of the tests prove satisfactory, the prospector is now ready to select a site for the test shafts or tunnel. The selection of this location will depend more or less on the topography of the country. If shafts are to be used at least two will be necessary and they should be at least 50 feet apart along the strike of the dike. One may be sunk at the natural place to open the dike, and the other should preferably be on higher ground. The shafts should be sunk not less than 25 feet into

the dike material or until the grade of the product is proven too low for commercial use. The most convenient small shaft is a 3 by 6 foot rectangular shaft, with the larger dimension across the dike. In sinking such a shaft, frequent and detailed record should be made of the appearance of the rock, and at intervals not exceeding 5 feet a 50-pound sample should be taken, and each of these samples crushed and quartered to a 5-pound sample after the manner described above. If more than one band of the dike is exposed in the shaft, each band should be sampled separately.

TESTING FELDSPAR.

If a feldspar as marketed was the powder of a pure mineral, the chemical analysis would doubtless afford a reliable means of checking the various shipments and thus insure uniform results from its use. As the feldspar of commerce necessarily contains other minerals which are associated with it and as the term "feldspar" is used to cover a broad range of chemical composition, the safest course for the user who desires uniformity of results is to determine the chemical composition and also the physical properties of the feldspar. For the methods of chemical analysis of feldspars, the reader is referred to United States Geological Survey Bulletin 422. The most important physical properties of feldspars are as follows: Temperature of deformation or fusion; rate of deformation; color when deformed and after complete fusion; shrinkage which its introduction imparts to a standard porcelain body.

TEMPERATURE OF FUSION.

Feldspars fuse at widely varying temperatures, owing to their great variation in chemical composition. When to this cause of variation is added that of an uncertain proportion of mineral other than feldspar, only a very general prediction can be made as to the physical and pyrometric properties of the mixture.

Fusion is a progressive process and covers the entire pyrophysical change from the solid to the fluid state. The only means of measuring the degree to which the process has attained is to determine the change in viscosity of the mass.

Measurement of the viscosity of fusion by means of physical apparatus is a difficult operation, and the apparatus at present available is not suitable for the use of any but highly trained experimentors. Fortunately for the practical man, as well as for the more highly trained investigator, there is a simple and accurate method of measuring viscosity change in the early stages of fusion which affords a reliable means of expressing the relative temperature and rate of fusion.

a Hillebrand, W. F., The analysis of silicate and carbonate rocks: U. S. Geol. Survey Bull. 422, 1910, 239 pp.

TESTING WITH STANDARD CONES.

The method referred to is standardization against standard pyrometric cones of known deformation temperature and rate of deformation. Deformation may be defined as the first visual evidence of fusion and indicates the reduction of viscosity to the point where the mass is no longer able to support its own weight. The rate of deformation evidences only the rate of fusion in its early stages, but by the method employed the possibility of error is reduced to the minimum.

The standard pyrometric cones are made from a series of mixtures of carefully tested and standardized materials similar to those employed in porcelain manufacture. They were first produced by Dr. Herman Seger under the patronage of the German Government. The temperature at which the cone deforms is regulated by its composition. Seger made a series of standard cones to fuse at intervals of 20° C. between 1,150° C. and 1,650° C., and numbered them 1 to 26. Later a series of standard cones for temperatures from 950° C.to 1,130° C. were devised by E. Cramer, an associate of Seger. The members of this series also fuse at temperature intervals of 20° C., and are numbered 010 to 01. A series of standard cones were later produced for temperature above 1,650° C., and these are known as Nos. 27 to 42, the latter being the fusing point of pure alumina. The temperature at which these pyrometric cones fuse is affected to a greater or less degree by the heat treatment which they receive, but with firing conditions constant they are reliable to within a few degrees.

The standard cones of the Seger series, which are the only ones in which the feldspar investigator is interested, are as follows:

Seger series of standard pyrometric cones.

Deformation temperature, °C. 1, 150

1, 170

1, 190

1,210

1, 230

1,250

1,270

1,290

Cone No.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

1, 310

1, 330

1,350

Cone No.

12.

13.

14.

15.

16.

17.

18.

19.

20.

26.

Deformation temperature, °C, 1, 370 1, 390 1,410

1, 430

1,450

1,470

1,490

1, 510

1,530
1, 650

Cones Nos. 21 to 25, inclusive, have proved so erratic in their behavior under commercial firing conditions that their manufacture and use have been abandoned both in Europe and America. The

« PreviousContinue »