V O L U M E 19, NO. 9
652 Table 1. Sar.iyle
P?O&
Bureau
Value
/-ingelements when pulverized in the customary mannrr and by a variant procedurc. hIORTAR
T h e samples xvere crushed in a mortar of the common Plat,tner type, KO. 17360 B of the Central Scientific Company. T h e inner cylindrical portion vas 3.5 em. in diameter and 1.6 cm.
high. The diameter of the pestle as 2.5 cm. The collar, which fitted both the mortar and pestle very snugly, had a height of 3.5 em. The mortar had been used prior to the present experiments and the face of the pestle, as n-ell as the floor of the mortar, was appreciably roughened by abrasion. The sides of the collar also shoved signs of abrasion. SAMPLES
Because of its purity and hardness, quartz is the logical test substance, b u t a sample of microcline was also crushed t o learn the effect of loner hardness and different crushing characteristics. The samples n-ere prepared as follows:
Quartz IA. Pieces of clear quartz crystal, up to approximatcly 1 cm. in diameter but mostly smaller, were tapped 20 times in the mortar with moderately heavy hammer blows. The crushed material was then sifted through a screen which consisted of a sheet of rag paper perforated with numerous holes 0.7 mm. in diameter. The material remaining on the screen was returned t o the mortar, tapped as before, and sifted, and new fragments of quartz were added when but little of the previous charge remained. After 200 tappings, 95% of the total sample had passed through the sieve. The remaining fraction was combined with the sifted portion and the whole mixed (weight 9 'grams). The powder thus obtained was not pure white, and small slivers of steel could be seen in it here and there under low magnifiration.
SEPTEMBER 1947
653
Quartz IB. This sample of clear quartz crystal was crushed without use of the collar of the mortar t o find whether less iron was introduced in this way. It was thought t h a t the elimination of abrasion caused b y powder getting between the collar and the pestle, on the one hand, and the mortar, on the other, might outweigh any incxeased abrasion arising from the sliding of the grains on the floor of the mortar. h cardboard shield with a n opening only slightly Tvider than the diameter of the pestle was used t o prevent loss from flying fragments. T h e powder was sifted after every 20 hammer strokes as before. After 150 tappings approximately 9570 of the sample had been passed through the screen. The coarse fraction was combined with the sifted portion and the whole mixed (weight 6.3 grams). Minute fragments of steel could be seen in the powder under magnification, but the powder seemed t o be slightly 13 hiter than IA.
Table I. Sample
Amounts of Aletals Introduced in C r u s h i n g Quartz and Feldspar in Steel 3Iortar re
AIn
Cr
v
SI
co
cu
f'pm
Quartz 11. This sample consisted of milky vein quartz. It was crushed in much the same manner as 1-1, but a screen with 1-mm. holes was used for sifting. Microcline. A specimen of amazonstone (Amelia Coui,thouse, Va.) apparently free from iron oside stains n-as selected as the most suitable feldspar material available. I n the preparation of sample A, a piece having a volume of about 1 cc. \vas crushed as described under Quartz 1-4, 60 blows of the hammer, with sifting after every 20 strokes, being required to reduce about 95% of the sample to powder t h a t would pass the screen. T h e whole v,-as mixed (weight 3.0 grams). T o obtain a correction for any acidsoluble iron compound's originally present in the microcline, small pieces which had nor been in contact with steel were ground in a n agate mortar, then treated with acid, and the iron in solution was determined. The original iron content of the sample thus found amounted to 30 p.p,m. Sample B was crushed without the use of the collar of the mort,ar. h total of 60 blows, n-ith sifting after every 20, was required to reduce approximately 2.5 grams of microcline t o powder. ANALYSIS
For the determination of the metallic iron in the various samples, several grams of each were heated with a misturc of hydrochloric and nitric acids and the colorimetric thiocyanate method was applied. Iron was also determined by the colorimetric mercaptoacetic acid method in the case of the microcline; the results were identical ivith those obtained with thiocyanate. I n order t o obtain sufficient iron from the mortar to cnable the subsidiary metals t o be determined, quartz powder was ground in the mortar with a rotary motion of the pestle, after the collar had been removed. I n a comparatively short time the powder became dark gray and contained enough iron foi, the purpose. The powder was heated with hydrochloric and nitric acids and made up to volume. The iron content of the solution was determined colorimetrically, so that the ratio of minor metal t o iron could be established. Suitable aliquots were taken for the det,ermination of the subsidiary metals by colorimetric methods. T h e periodate method was used for manganese after removal of chlorides. Chromium was osidizrd with sodium peroxide and determined with diphenylcarbazide after suitable treatment. Vanadium was determined as phosphotungstovanadate after removal of iron. Sickel was separated from iron by chloroform extraction of its dimethylglyoxime compound from ammoniacal citrate solution and determined with diniethylglyoxime and bromine. Cobalt was separated from iron by dithizone extraction and finally determined as the'complex thiocyanate in acetone solution. Copper was determined n-ith sodium diethyldithiocarbamate in citrate medium b>-carbon tetrachloride extraction. T h e results (calculated from the iron ratio) are reported opposite
Quartz Id in Table I. Zinc and lead were not determined b e cause significant amounts of these are not t o he expected in %teel. coscLusIoss
It seems reasonable to suppose t h a t the amount of iron introduced in crushing most silicate rocks !vi11 not exceed that introduced in crushing quartz. The addition of 0.02 or 0.03% of metallic iron w i l h o t significantly affect the figure for the total iron content of a rock, but it will.appreciably raise the value for ferrous iron and therewith lower the value for ferric iron (the latter partly compensated), since 0.03% metallic iron cvrrcqmnds to almost 0.1% ferrous iron. The conditions of crudiing i n the present experiments are probably more favorable than those ordinarily obtaining in keeping the quantitj- of introtlurcd iron not hi. dcairatile low, because a coarse screen was uscd. It 1%-ould t o crush more coarsely than in the present work herau3e of the difficulty of subsequent grinding in agate without loss from flying particles. Even the sifted quartz obtained above trnds to be ejected t o some extent in the initial stage of grinding in an agate mortar, and it is best t o use a cardboard shield a t firet t o prcvcnt such loss. So far as the trace constituents of igneous rocks are cwnceimc.d, the amounts of these contributed by the Plattner mortar will for the most part be small enough t o neglect, if the experiments here described can be' considered representative. An iricrrase of 2 p.p.m. in manganese content is negligibly small compared to the amounts usually present, although special cares cwuld conceivably be encountered in which a quantity of thi.: order might play a role. Only in highly silicic rocks is the :iic:kt4 m i t e n t likely to be less than 1 p.p.ni., so t h a t the introduction of a few tenths of 1 p.p,m. is usually of no importance. The same conclusion holds for copper. The chromium conTerit of the mortar usrd in the present 11-ork is higher than desirahlt,, f i l l he addition of 0.5 p.p.m. of foreign chromium nould be ot inoment when silicic rocks are involved. By constructing thi. niurtar of steel scllcctcd for Ion- cliroiiiium, nickel, and copper c o u t e n t , the amount of contamination of the sample can be kept iir~yligibly 11 csacting work i n the deterniination ot trace elcments according to ;)rc-eiit-day rquirements. Tri tlii. connection attcntion should 1 1 ~called to the po*sikJlil (j['('\ii':'f'ileeof unespccted c~lenir~rit; i n steel mortars. Ranltama I 2 ) nliLtitiow the presence of columbium and tantalum in a niortat' ot t l i t . 1:llis type. I t appcars that thew is less contamimtion Ly iron in ca:,ushing in a steel mortar of the Plattner type if the collar is not usrld. Apparently considerable abrac-ion results when the pov-dt)r comes into contact with the soft and tightly fitting collar. Cru!hing can be done more efficiently and rapidly if the collar is not used, since there is then less tendency for the powder ,to cakr. and the material is readily brought to the center of the floor by tapping shield, such as the mortar a t intervals during the crushing. can be improvised from a piece of cardboard, must however, be used to prevent loss from flying fragments and, to a leseer ( w e n t , from dusting. S o doubt a better design for the Plattner-type mortar than the one now in use can be devised. ACKNOWLEDGIIENT
The writer is indebted to S. S. Goldich of the Bureau of Economic Geology, L-niver.5ity of Tesas, for reading and commenting on the manuscript. Goldich states that his experienctx also has been that the use of the collar in the Plattner mortar iriri,c,ases the amount of iron introduced in crushing. LITERATURE CITED
(1) Otto, H., -1Iinei.uZog. P e f r o g . -lIlff., 47, 126 (1930). ( 2 ) Rankamn, K., Bull. comm. g802. F i n l a n d e , No. 133, 11 (19441
