[Aztlan] Ocher

[Benjamin Carter]

Bruce,

Thank you for the 'real' information. It is very clear that many of 
these terms are quite amorphous from both a descriptive as well as 
chemical perspective. I was completely in the dark about goethite and 
it's relationship to limonite.

Also, looking at all of the photos on mindat.org indicates how variable 
any one mineral can be. For example, check out hematite at 
http://www.mindat.org/min-1856.html . There are 737 photos to look through.

Thanks again,
Ben Carter, Ph.D., RPA


Bruce Rogers wrote:
> Listeros,
>
> A pocket guide to more than you ever wanted to know about reddish pigments:
>
> Ocher:
> (a) an earthy, pulverent [crumbly, dusty, powdery] red, yellow, or 
> brown iron oxide that is used as a pigment; e.g. "yellow or brown 
> ocher" (limonite) and "red ocher" (hematite).  Also any of various 
> clays colored by iron oxides [be careful here - fluvic and humic soil 
> acids also tint clays in ocher colors].
>
> (b) A similar earthy and pulverent metallic oxide used as a pigment; 
> e.g. "antimony ocher" (stibnite and cervantite), "red lead" (massicot 
> and litharge), and "tungstic ocher" (tungstite and ferrotungstite).
>
> (Bates, Robert L, and Jackson, Julia A., eds., Glossary of geology, 
> second edition: Falls Church, VA, American Geological Inst., p. 431.)
>
> Hematite - A common iron mineral: alpha-Fe2O3 [iron oxide]. ... 
> Hematite occurs in splendent, metallic-looking, steel-gray or 
> iron-black rhombohedral crystals, in reniform masses, fibrous 
> aggregates, or in deep red (Munsell 10 R 3/6] or red-brown (Munsell 
> 2.5 YR 4/6) earthy forms ...  (Bates & Jackson, p. 288).  Hematite 
> can be usually identified in the field by its dull, but intense red 
> to iron-black shiny color and, when scraped across an unglazed 
> porcelain plate or back of a uncolored tile, its bright red streak. 
> Most coarsely crystalline hematite is lustrous steel gray or black; 
> earthy varieties are dull red (Munsell 10R 3/6).
>
> Goethite - A yellowish, reddish, or brownish-black mineral: alpha-FeO 
> (OH). It is trimorphous [three different crystal structures with the 
> same chemical formula] with lepidocrocite and akaganeite.  Goethite 
> is the commonest constituent of many forms of natural "rust" or of 
> "limonite," and it occurs, especially, as a weathering product in the 
> gossans [rusty-colored iron oxide weathering caps of many mineral 
> deposits] of sulfide bearing ore deposits.  (Bates & Jackson, p. 
> 267).  Goethite is by far the most common ocher mineral.
>
> Limonite - A general field term for a mixture of brownish, amorphous, 
> naturally occurring hydrous iron oxides whose real identity is 
> uncertain, pending identification in the lab.  Limonite ... is now 
> considered ... to consist of any of several common hydroxides 
> (commonly goethite) or a mixture of several minerals such as 
> hematite, goethite, or lepidocrocite ...  It is a common secondary 
> material formed by oxidation (weathering) of iron or iron-bearing 
> minerals, and it may also be formed as an inorganic or biogenic 
> precipitate on bogs, springs, or marine deposits; it occurs as 
> coatings (such as ordinary rust), as loose or dense earthy masses, as 
> pseudomorphs [primary minerals replaced with other secondary mineral 
> that retain the original crystal shapes] after other iron minerals, 
> and a variety of stalactitic, fibrous, reniform [kidney-shaped], 
> botryodial [bunch of grapes-shaped],  or mammilary [breast-shaped] 
> forms, and it represents the coloring material of yellow clays an 
> soils.  Limonite is commonly dark brown [Munsell 7.5 YR 3/3 or 4/3] 
> or yellowish brown Munsell 10 YR 5/4-8], but may be yellow, red, or 
> nearly black; it is a minor ore of iron.  (Bates & Jackson, p. 360)
>
>
> So in general field use by geologists, mineralogists, and soil 
> scientists, ocher usually refers to earthy, red-, yellow-, and/or 
> brown-colored "stuff."  The color usually is the result of oxidation 
> (rusting) of various iron minerals.  The color easily combines with 
> light-colored clay minerals to form "terra rosa" soils (literally 
> "red earth").  Generally, steely gray or lustrous black minerals like 
> magnetite (common beach or river bottom "black sand") are not 
> considered ocherous materials.
>
> Cheers,
> Bruce Rogers, earth scientist on a good day
>
>
>
>
>
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