V O L U M E 26, NO. 4, A P R I L 1 9 5 4
717
the other oxides to such a n extent that this method is not applicable; identification made with other methods should then be confirmed by using the etching techniques. EFFECT OF VARIABLE OXYGEN CONTENT ON REFLECTIVITY
Hematite can usually be identified by its relatively great reflectivity: in partly reduced 01%~.metdlie iron is the only commonly encountered phase that has a reflectivity greater than that
Figure 3.
COMPARISON O F MICROSCOPIC AND X-RAY METHODS
The use of x-ray diffraction techniques for identifying iron oxide phases is limited because each phase has a concentration threshold helow which' i t cannot be detected by x-ray methods. Furthermore, silicate pha'es in the ore gangue may mask mall concentrations of iron oxide when x-ray methods are used.
Same Ore Specimen a f t e r Scratching
~-..
Scratch produced by drawing sharppointed -.>,.
flectivity than does magnetite; however, the difftrence cannot always be detected by eye. In polarized light, hematite exhibits anisotropy, hut magnetite and wustite are isotropic. After the phases have been identified, their concentrations may be determined by the pointdaunt method (1).
......
->
~~
~
~ : - ~ ~~
- . ~5. Same - ~ Ore Specimen ~ after Light Etching with S a t u r a t e d Solution of Alcoholic S t a n n o u s Chloride Light etching aeeenfu~fesFeO
of hematite. The reflectivity of magnetitt. II U B U W ~ LWB ~ an that of hematite. However, it is possible for magnetite to contain oxygen in excess of its stoichiometrical composition, which results in B proportionate increase in reflectivity. Such an excess of oxygen ismost likely to occur in ores that have been wholly or partly fused and rapidly cooled; all or part of the excess oxygen ii the magnetite is retzinined in solution during fast cooling an,d makes the reflectivity of the magnetite similar to that of heme tite.
"
4 e.101e examinatinn nf ranirllu-~onlerl nm mill ..___ _._ Ir___i _.I ..... ~
..
t > a n s I l x r vhnr ".V I_"
hematite arranged in a Widmanstatten pattern within the magnetite grab. If the fused ore has been slowly cooled, hematit? will be prevalent, and the reflectivity of the magnetite will he less than that of the hematite. Wustite usually hrts slightly less re-
ens. Oriainal magnification X 125, reduced %/a in reproduction
In this laboratory, reducibility studies on iron ores have shown that some iron oxide remains even after 7 hours of reduction in hydrogen a t 800" C. Microscopic examinations, Table I, canfirmed this finding, although x-ray analysis did not show the pres?me of iron oxide after only 15 minutes of hydrogen reduction a t 8000 c. Occasionally, when the gritin Bise of the phases is extremely s m d and the phases are intermingled, microscopic methods may not be adequate and x-ray analysis or chemical analysis must be used. Usually, microscopic m&ods Kill movide the desired informatio n. LITERATURE CITED (1) Chayes, F., Am. Minealogist. 34 (1/2), 1-11 (1949).
(2) Cooke,
5. R. B., and Ban, T. E., Tram. Am.
R n ms.,
Znst. Mining Met
193, 1053-8 (1952).
RECE~VED for review Mkroh 28. 196.3. Accepted Janunry 27, 1RJ4. Presented before the Division of Indualrial and Engineering Chemistry. Symposium on Recent Developments in Ceramics and Glass. st the 123rd Meetinc of the I r n n i c ~ aC x ~ x r:AL c Socrsru. Lo8 Angeles, Calif. Other pspen ;hed ~nthe Sanusry 1954 iswe of Industrial a m1 in the S Y ~ P D S ~were~ublir U ~ En~ineeringChemiairy.
Determination of lake KIWIS-VUI
Figure 4.
-..
Same Ore Specimen w i t h D i a m o n d Hardness 1 n d e n t a t i o n s
Indentations made eores8 FeO and Fe104areas. Original magnification X125, mdiaced s/& in repmdnction
IGGIIUII
I n the article on "Determination of Calcium and Magnesium in Lake Waters" [ANAL. CnEx., 26, 347 (1954)l the formula a t the bottom of Table I, page 348, should read.
%,"s = -
100
1: 1
(C
- c)2
cdk=r
V. W. MELOCHE
