Rapid Determination of Quartz in Feldspar G . H. MCISTYRE AND M. BOZSIN, Ferro Enamel Corporation, Cleveland, Ohio
I
N THE chemical analysis of feldspar, all of the silica is
determined as such, but no determination is made of the amount of quartz. As quartz and feldspar are major ingredients in vitreous enamels, and the ratio betv-een the combined silica and quartz in feldspar is variable, it is important to determine the quantity of quartz present in feldspar in order to ensure uniformity of enamel composition
OCULAR 125 OBJECTIVE 101 16 MM STANDARD SAMPLE ROD MOUNTED IN GLYCEROL 3/4 INCH CIRCULAR WATCH GLASSES
-
N
STAKDARDS. Before routine determinations can be made, a set of standards must be prepared in order to establish points on a graph which can be used in all subsequent determinations. It is best to select a feldspar that is known to have a low quartz content, for making up the TTorking standards. Five samples containing feldspar with quartz added in different percentages usually make u p the standards. All materials used must be ground so that a t least 95 per cent passes a 200-mesh sieve. Feldspar and quartz must be clean and free from impurities such as iron and excessive mica. Quartz of a t least. 99.70 per cent silica grade should he used. The standard mixes are made in the following proportions:
l 8o0o I
/
I
I
I
03
06
09
I
I 120
150
I80
210
on the technique of detecting rice starch adulteration with potato starch. Booze and Klein (2) developed a method basically similar to this one, but differing in technique. Their description does not cover the preparation of standards and the exact technique used in preparationof the slide. Thompson ( 7 ) determined free silica in feldspars by the linear intercept method, from these calculating the weight of auartz present. Insley ( 4 ) also used the line& intercegt method on thin sections prepared from fusions of the feldspar. Parmelee and ICIcVay (6) used an adaptation of this same method. McCaughey ( 5 ) determined quartz in crude feldspar by indices of refraction metlioda and the proportions determined by counting the grains.
240
Sample No.
QUARTZ PARTICLES PER UNIT AREA
FIGURE1. TYPICAL ST.4NDARD C H a R T FOR TRAXSLATISG QUARTZ GRAINC O U N T TO PERCENTAGE COMPOsITlON IK FUSED FELDsP.4R
Watkins ( 8 ) has discussed the relation of quartz in feldspar
to chipping of enamels, and gives a method for calculating the quartz from a complete chemical analysis of the feldspar. This is a long and difficult method and does not lend itself to rapid control of raw materials in glass and enamel manufacturing processes.
Method The method described for the determination of quartz in feldspars is rapid and requires no special apparatus for sample preparation and grain diameter measurements. It has been standard in the analytical laboratories of the Ferro Enamel Corporation for over 10 years and has been proved to be practical. It is based upon the principle that unfused quartz is anisotropic and that when a powdered vitreous mixture is viewed under a polarizing microscope with crossed Kicols. the number of quartz grains in a definite area may be counted. These will appear light or colored, whereas the glass will be dark. Quartz grains which are in the extinction position will remain dark when so examined, but as the standards are examined in the same manner, the error is nullified. Chamot and Mason (3)referred biiefly to the method as being fundamentally based
FIGURE 2 PALO MYERSGAS-FIRED FURNACE 326
1 2 3
+
Quartz
Feldspar Grains
Quartz Gravis 50 100 I50 200 250
Each combination must be tumbled thoroughly (preferably in a hall mill) for a t least a n hour, to provide a homogeneous mixture. F U S I ~BGTTOM. K To about 100 grams of each mixture contained in a casserole, add 10 ml. of 1 per cent gum arabic, gum tragacanth, or gelatin solution. Mix the gum thoroughly with the sample by means of a spatula. The gum acts as a binder which serves to hold the mixture together in a mold. Pack the resulting mixture into a 15-ml. porcelain crucible until level, scrape off even with the top, and invert on a small porcelain plate. Tap the crucible gently to release the fusion button from the crucible mold.
AS.kLY TICAL EDITION
JUNE 13, 1910
327
TABLEI. DETERMISATION OF QUARTZ Feldspar
Si02
AlzOa
CaO
%
%
%
A1203.6Si02
Kz0
IC90 Ah08.6SiO?
--Mineral caq. .ilzOs.6siO?
%
%
%
70
11.00 10 4 1 '3 07
8.7 6 4 9.0
88.65 82.65 83,42
2.10 1.82 1.23
2.36
2.0
97.00 95.00 82.20
Sat0
%
Feldspar mill 2282 Feldspar mill 2513 l'irginia 8922 (high quartz)
68.11 69.03 68.98
17.75 0 . 2 5 19.40 0.22 1 8 . 8 1 0 15
Virginia soda spar, 3979
68.59
Derry spar (Canada) Carolina spar
65.85 68.87
1 8 . 1 0 0 25 10.00 1IgO 0.82 18.82 0.32 2.46 1 7 . 7 6 0.032 3.02
2.52 2.38 4 13
12.50 Few particles 9.31 9.2
FESION AKD BETTON FIGCRE3. FELDSPAR Prepare a fusion button of each of the five mixtures. Place the buttons in an oven for an hour at 110" C. to allow all free moisture to escape. After the initial drying, all five samples must be fused. For this purpose a Globar element furnace or a Palo gas furnace may be used. Heat the samples uniformly, gradually increasing the temperature until 1370" C. (2500" F.) is reached, and maintain this temperature for 15 minutes.
HEATTRESTMEXT. The following schedule is follomd during the heat treatment: Min
0 15 30
45 60
.
O
F.
1500 1700 1900 2100 2240
Mtn. 75 go 95 100 105
O
NalO
Quartz Determined by Count Method
F.
2380 2500 2500 2500 2500
During this heat treatment all of the feldspar fuses t o a translucent glass, while the quartz remains unaffected.
CompositionKaolinite
Quartz (Calcd
)
CI
