[Aztlan] Ocher

[Bruce Rogers]

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