Observation, Description and Interpretation of Ironstone, Phosphatic Rocks and Bauxite

 

Minerals which you may encounter during this lab. Use the web Athena Minerals or a mineralogy text book to find the physical and optical properties of each of these.

Hematite Fe2O3 Opaque in thin section, red streak, reddish or silvery (metallic)
Magnetite Fe3O4 Opaque in thin section, black streak, dark submetallic luster, magnetic
Pyrite FeS2 Opaque in thin section but with a brassy color in reflected light. Yellow metallic luster, common cubic crystal habits.
Marcasite FeS2 Opaque in thin section, more whitish than brassy like pyrite in reflected light. Greenish metallic luster. Oxidizes rapidly when exposed to air.
Pyrrhotite FeS Opaque in thin section, brownish bronze with a black streak. Magnetic.
Siderite FeCO3 High birefringence, light to dark brown with a vitreous luster, perfect rhombohedral cleavage. Will not stain with potassium ferrocyanide blue.
Limonite or goethite FeOOH Red in thin section but could appear opaque if the thin section is thick. Rust like in handsample. Yellow-brown streak.
Glauconite K2(Mg,Fe)2Al6(Si4O10)3(OH)12 Green in thin section and hand sample, cryptocrystalline.
Chamosite (Fe5Al(Si3Al)O10(OH)8 Brown in thin section and hand sample, cryptocrystalline.
Greenalite Fe3Si2O5(OH)4 Cryptocrystalline
Minnesotaitie Fe3Si4O10(OH)2  
Apatite Ca5(PO4)3(F,OH,Cl) Usually brown, yellowish or colorless in plain light. Sedimentary apatite mostly cryptocrystalline. Several subvarieties : fluorapatite, hydroxylapatite, chlorapatite, dahllite. See callophane.
Gibbsite Al(OH)3 Cryptocrystalline
Diaspore AlO(OH) Tabular or fibrous, usually small crystals
Boehmite AlO(OH) Cryptocrystalline

A trick that can be used to aid in the identification of opaque minerals is to view them in reflected light. There are special petrographic microscopes for this, which have a light source that illuminates the thin section from directly above. If you do not have this type of microscope  you can do sort of a poor man's version by first blocking off the substage illumination and then taking a flash light and shining it down as steeply as possible unto the thin section.

Cyrptocrystalline material really needs to be evaluated by x-ray diffraction in order to make a reliable identification. The trouble there is trying to separate out enough of a single mineral to avoid interference from the others. Sometimes you must just rely on simple intuition and a little of principle of least astonishment. Learn what minerals are associated with what.

Ironstone

Some terms that you may want to look up in the Glossary of Geology include: ferricrete, ferrilith, ferrite, ferroan, gossan, iron formation, iron range, ironstone cap, iron ore, jaspilite, jasper, jasperization, jasperoid, specularite, and taconite.

There is no formal classification for ironstone. What I suggest is you perceive that it contains 15% or more Fe by virtue of its heft (weight in hand) call it an ironstone. If you can identify the major iron bearing mineral than use that name as a modifier of ironstone (i.e. Hemititic ironstone, limonitic ironstone, pyritic ironstone). If the mineral is siderite you should use sideritic not siderose. The former refers to the mineral siderite while the latter refers to it having an iron content and is a synonym of ferruginous. If there is a major textural feature add that also as a modifier. (i.e. oolitic chamositic ironstone, hematitic banded ironstone). If you do not think that the sample is high in iron concentration (<15%) then try to apply a sandstone, mudstone, or carbonate name to the rock with the iron mineral as a modifier (i.e. pyritic quartz arenite, hematitic fossiliferous packstone, glauconite arenite). If you are still stumped and it is red or rusty colored then just call it ferruginous shale, or ferruginous sandstone.

When you are examining ironstone you should be trying to figure out first why the iron is there. Iron is not very soluble in the modern weathering environment but it was in the distant past. Is you rock young or old (as old in the sense of Archean)? If old, then the iron could be an allochemical component of the rock just like bioclasts are allochemical components of limestone. If young, then was the depositional environment oxygen depleted (low Eh)? If it formed in an oxygen depleted (anoxic or dysoxic) depositional sedimentary environment then it too could be an allochemical component or an early or syndepositional diagenetic product. If the depositional environment was not oxygen depleted then the iron must have resulted from some post-depositional process. Then once that is clear then solve the problem of how it got to the site of the rock and from where did it come. But, sometimes you must first figure out what has happened to the iron or iron bearing components since they formed regardless of how they formed. What is limonite today probably was not limonite at first. To figure any of this out you have to look for the clues, do you detective work!

Exercise#1: Please examine each of the following hand samples and thin sections and do your best to observe, describe, and interpret.

Ironstone or Iron-bearing Sedimentary Rocks and Their Associates

Hand Samples

Thin Sections

Phosphorite or Phosphatic Sedimentary Rocks

Hand Samples

Thin Sections

Al-Rich Sedimentary Rocks or Bauxite

Hand Sample

Thin Section

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