Mica Visible Spectra (generally 350 - 2500 nm)
polarizations: (a = alpha direction; b = beta direction; g = gamma
direction)
Images of representative mica spectra
Micas usually owe their color to Fe, Mn, Ti, and Cr. Iron
contributes as Fe2+, Fe3+,
Fe2+ - Fe3+ intervalence
charge transfer, and as Fe2+ - Ti4+
intervalence charge transfer. Manganese contributes weakly through Mn2+,
and strongly through Mn3+ and associated color
centers induced through natural radiation damage. Chromium in the 3+
oxidation state contributes an intense green color to some micas.
Manganese-containing micas
-
pink lepidolite spectrum image;
Lepidolite, GRR 678, Himalaya Mine, San Diego County, California, 1.0
mm thick with Mn3+ in octahedral sites
dominating the spectrum. Data Files: a ; g
- brownish-pink lepidolite spectrum image;
Lepidolite, Coolgardie, Western Australia, 1.661 mm thick with Mn3+
in octahedral sites dominating the spectrum. Data File: cleavage
-
reddish-purple muscovite spectrum image;
Muscovite, GRR 727, Minas Gerais, Brazil, alurgite variety, 0.5 mm
thick with Mn3+ in octahedral sites dominating
the spectrum. Data Files: a ;
b ; g
-
pink muscovite spectrum image;
Muscovite, GRR 805, Harding Mine, New Mexico, USA, 2.0 mm thick. Data
Files: a ; b ;
g
-
orange-brown phlogopite spectrum image;
Phlogopite, GRR 728, Langban, Sweden, manganophyllite variety, 0.1 mm
thick. Data Files: a ; g
Iron-containing micas
-
medium-green phlogopite spectrum image; Phlogopite, GRR
1575, Ontario, Canada, 1.897 mm thick, showing the Fe2+ - Fe3+ intervalence
charge transfer neae 700 nm, the Fe2+
absorption near 900 nm, and the OH overtone near 940 nm. Data
Files: perpendicular to cleavage
plane, 24K; parallel to the
cleavage plane
-
red-brown phlogopite spectrum image;
Phlogopite, KB5-2, Kimberley, South Africa, with reverse pleochroism
from Fe3+ in the tetrahderal site, 0.03 mm
thick. Reference: Farmer and Boettcher (1981) Amer. Mineral 66,
1154-1163. Reverse pleochroism means that the most intense absorption
occurs when the electric vector of the incident polarized light is
parallel to the c-axis, opposite to the case for
most common micas. Data Files:
a ; b
- mica spectrum,
Beebe Hole Mine, Tule Mountain, San Diego County, California, 2.66 mm
thick cleavage. Polarized perpendicular to the cleavage slab showing the
Fe2+ bands.
-
Chromium-containing micas
-
green muscovite spectrum image; Muscovite, GRR 1461, near
Randsburg, Kern County, California, 0.493 mm thick, showing typical Cr3+
absorption near 600 nm. Data Files:
cleavage plane
- another slab, 0.180 mm thick cleavage slab, unpolarized. Data File
-
brown roscoelite spectrum image; Roscoelite, GRR 1927,
Stuckslager Mine, Coloma, El Dorado County, California, 0.040 mm thick,
showing Cr3+ absorption near 600 nm. Data Files:
cleavage plane
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