Birmingham, Alabama Clinton-type Sedimentary Iron Ores - Phosphorus Problem



An exposure of the Big Seam of hematitic (Hm; red) iron ore in the Big Cut at Birmingham, Alabama. The stratiform ore is contained within sandstone (Ss; gray) units of Silurian age.



First of a sequence of three photomicrographs illustrating different microscopic techniques of studying the Birmingham iron ores. Normal reflected light view showing the typical oolitic texture of the ores. Only part of the very fine-grained hematite (Hm; white) has taken an adequate polish to be studied by reflected light microscopy. Much of the hematite has not polished. Most of the gangue is calcite (Cal; dark gray). Two clastic quartz grains (Qz) are distinguishable due to their high relief. Ore microscopy, reflected light, low magnification.



Reflected light view using, in addition, strong inclined reflected light. Hematite (red to bluish black) has preferentially replaced bryozoan fossil fragments in this fossiliferous portion of the iron ores. Sparry calcite (Cal; white) has cemented the fossil fragments. Birmingham, Alabama iron ores. Ore microscopy, reflected light plus strong inclined reflected light, low magnification.



Cathodoluminescence (CL) microscopy (CLM) view of Birmingham, Alabama ores. CLM reveals the details of the textures of the calcitic (Cal; bright yellow) bryozoan and other fossil (echinoid, upper right) fragments. Quartz (Qz) exhibits dull purple to brown CL. Hematite, which does not CL, is black. CLM, low magnification (20X in original slide).



Single grain of collophane (Col; light bluish grey) with a nucleus in which a fossil fragment has been replaced by collophane and it was subsequently coated by layers of chemically precipitated collophane. Calcitic (Cal; yellow) to dolomitic (orange) fossil fragments and clastic quartz (Qz; dull purple) also are contained in the hematite (black) matrix. One of the major features of the Birmingham iron ores is their high content of deleterious phosphorus. This chemical characteristic, together with their fine-grained nature, has led to the disuse of those ores. If the phosphorus could be reduced by some means of benefication, the ores would be more attractive as a raw material source. Collophane grains of this size should be ameanable to separation by conventional beneficiation techniques. Birmingham, Alabama iron ores. Cathodoluminescence microscopy, moderate magnification (100X in original slide).



CLM also reveals the presence of abundant very fine-grained collophane (bright grey) that would not be ameanable to conventional means of separation by beneficiation. Calcitic bryozoan fossil fragments (Cal; yellow), large clastic quartz (Qz; dull purplish brown) grains, and three small clastic potash feldspar (bright blue) grains are present in the fine-grained hematitic iron matrix (Hm; black). Birmingham, Alabama iron ores. Cathodoluminescence microscopy, moderate magnification (100X in original slide).



Another CLM view showing abundant very fine-grained collophane (Col; bright grey) that would not be expected to respond to conventional means of separation by beneficiation. Calcitic (Cal; yellow) bryozoan fossil and quartz (Qz; dull purple) fragments form nuclei in hematite (black) ooids. Small clastic potash feldspar (bright blue) grains also are present. Birmingham, Alabama iron ores. Cathodoluminescence microscopy, moderate magnification (100X in original slide).