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DOCUMENTATION_FORMAT: MINERAL
SAMPLE_ID: HS36.3, HS36.3B
MINERAL_TYPE: Hydroxide
MINERAL: Goethite
FORMULA: alpha-FeO(OH)
FORMULA_HTML: alpha-FeO(OH)
COLLECTION_LOCALITY: Biwabik, Minn
ORIGINAL_DONOR: Hunt and Salisbury Collection
CURRENT_SAMPLE_LOCATION: USGS Denver Spectroscopy Laboratory
ULTIMATE_SAMPLE_LOCATION: USGS Denver Spectroscopy Laboratory
SAMPLE_DESCRIPTION:
Polymorphous with Akaganeite, Feroxyhyte, and Lepidocrocite.
Original spectrum published in: Hunt, G.R., J.W. Salisbury, and C.J. Lenhoff, 1971, Visible and near-infrared spectra of minerals and rocks: III. Oxides and hydroxides. Modern Geology, v. 2, p. 195-205.
With the note:"Like cuprite, goethite displays trans-opaque behavior, but does so at a slightly shorter wavelength (near 0.55µm). The ferric ion itself produces the feature near 0.9, and the fall-off short of 0.55µm is explained as being due to the presence of a conduction band with a quite well-resolved absorption edge, which is typical of the trans-opaque iron oxides. It should be noted in particular that goethite displays no hydroxyl bands in the near-infrared, despite its traditional formula, FeO-OH. Like diaspore (see above), the pure mineral should contain no free hydroxyl groups (Ewing, 1935)."
Grain size fractions are indicated by the extension after the sample number:
.3B = 74-250 µm
.3 = 74-250 µm (compared to .3B, a grinding process was used which resulted in trace metal shavings from grinder)
IMAGE_OF_SAMPLE:
NO PHOTO
END_SAMPLE_DESCRIPTION.
XRD_ANALYSIS:
40 kV - 30 mA, 7.3-9.5 keV
File: gotht36_mdi, *.out (smear mount on quartz plate)
References: Huebner's reference patterns
Found: goethite, minor quartz, unidentified reflection at
Comment: Goethite reflections are moderately sharp, indicating moderate degree of
crystallinity. Quartz (primarily the (100) reflection) is very sharp but weak.
END_XRD_ANALYSIS.
COMPOSITIONAL_ANALYSIS_TYPE: None # XRF, EPMA, ICP(Trace), WChem
COMPOSITION_TRACE: None
COMPOSITION_DISCUSSION:
None.
END_COMPOSITION_DISCUSSION.
MICROSCOPIC_EXAMINATION:
mode:
90 vol% fibrous goethite
10 vol% quartz or other clear mineral
trace paper fibre
trace metal shavings from grinder
Bimodal grain size distribution:
mode 1: average grain size = 300 µm @ 98 vol%
mode 2: average grain size = 10 µm @ 2 vol%
average grain size = 297 µm
*above grain size for goethite only; average grain size quartz = 150 µm
Fibrous brown grains (prismatic?), length slow, biaxial, high relief. Translucent red in plain polarized light. Grains are slightly pleochroic. These properties are consistent with goethite. Sample should be cleaned of iron shavings with a magnet. G. Swayze.
END_MICROSCOPIC_EXAMINATION.
SPECTROSCOPIC_DISCUSSION:
END_SPECTROSCOPIC_DISCUSSION.
SPECTRAL_PURITY: 1b2b3c4b # 1= 0.2-3, 2= 1.5-6, 3= 6-25, 4= 20-150 microns