| Author | Message | |||
     Michael C Mound (mike) Senior Member Username: mike Post Number: 63 Registered: 7-2007 |
Very apt. As a geologist/mineralogist, I should add...the term "sand" is correctly applied to the size of the reduced size of rock materials, and has absolutely nothing to do with its mineralogical or compositional content. Boulders, cobbles, pebbles, granules, sand, silt, and clay are the progressively comminuted forms of the products of chemical, mechanical, and even wind erosion. Thus, it should come as no surprise that the spectral characteristics should vary with the provenance of the source rocks. | |||
| Pierre Dardenne (dardenne) Senior Member Username: dardenne Post Number: 65 Registered: 3-2002 |
Dear All, I think Brian got the answer. In fact these spectra are pixels from HS images (Burgermetrics). We scanned kilos of beach sands to find small pieces of plastic. These pixels were wrongly classified and I was wondering what it was. It is not surprising to get silicates (muscovite) in sands. Now I have the answer thanks to your knowledge. Thanks Pierre WIKI: The composition of sand is highly variable, depending on the local rock sources and conditions. The bright white sands found in tropical and subtropical coastal settings are eroded limestone and may contain coral and shell fragments in addition to other organic or organically derived fragmental material.[2] The gypsum sand dunes of the White Sands National Monument in New Mexico are famous for their bright, white color. Arkose is a sand or sandstone with considerable feldspar content, derived from the weathering and erosion of a (usually nearby) granitic rock outcrop. Some sands contain magnetite, chlorite, glauconite or gypsum. Sands rich in magnetite are dark to black in color, as are sands derived from volcanic basalts and obsidian. Chlorite-glauconite bearing sands are typically green in color, as are sands derived from basaltic (lava) with a high olivine content. Many sands, especially those found extensively in Southern Europe, have iron impurities within the quartz crystals of the sand, giving a deep yellow color. Sand deposits in some areas contain garnets and other resistant minerals, including some small gemstones.
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| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 459 Registered: 9-2001 |
Brian - yes, I know that bulk sapphire wouldn't. But niether would bulk low -OH SiO2. If the SiO2 is very finely divided, however, you could get enough surface Si-OH to measure; I know that because we did it when I was a post-doc at NYU. I was wondering if a similar effect could occur in the sapphire, and also shift the bands enough to see what we observe? \o/ /_\ | |||
| Brian Curtiss (briancurtiss) New member Username: briancurtiss Post Number: 3 Registered: 6-2006 |
Sapphire (the gem variety of corundum) doesn't have any OH in the structure so it doesn't have any features near 2200 nm. The blue color in sapphires comes from charge transfer features of the trace amounts of iron or titanium substituting for aluminum in the structure. Here is a plot of a couple of typical muscovite spectra:  | |||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 458 Registered: 9-2001 |
Brian - interesting possibility. What are the chances that it's sapphire? \o/ /_\ | |||
| Karl Norris (knnirs) Senior Member Username: knnirs Post Number: 59 Registered: 8-2009 |
venkynir, Please let me know what is wrong with high quality spectra collected 30 years ago. For any interested parties, I have a file of 424 high quality spectra collected on a digitized Cary14 with a resolution of 7 nm, a point spacing of 16 Angstroms, and a wavelength range of 1000-2636.8 nm. Most of the spectra are in log(1/R), but the liquids are with an unspecified pathlength in transmission. I have these spectra in a Jcamp file which can be available by e-mail from [email protected]. I did search this file, and found similar spectra, but no match to Pierre's spectrum. | |||
| Brian Curtiss (briancurtiss) New member Username: briancurtiss Post Number: 2 Registered: 6-2006 |
The peaks near 2135 and 2200 nm are likely associated with Al-OH. Given the presence of an OH feature near 1425 nm and the lack of a strong feature near 1900 nm, it narrows the candidates down to a layered alumino-silicates without water in the structure. That rules out clays like those in the illite and smectite groups. That leaves kaolinite group clays and muscovite mica as potential candidates. A quick look at spectra from those minerals seems to give the best match with muscovite. | |||
| venkatarman (venkynir) Senior Member Username: venkynir Post Number: 139 Registered: 3-2004 |
Hello Tony Davies , Howard Mark & Karl ; Our experince shows that if the samples contain Higher moisture it will reflect in 1420-1430 nm band : examples ; we measured bagasse moisture ( high percentage moisture ) that help boiler optimization . In the case paper dry end we found 1930-1940 nm to be good for low percentage moisture . More ove later case the surface temperature of sample is high and humidity also plays it role. Karl we could not judge any thing specificly from spectral data taken 30 years before . | |||
| Lois Weyer (lois_weyer) Senior Member Username: lois_weyer Post Number: 44 Registered: 7-2002 |
Silanol bands appear in optical fibers or other optical components, in case you hadn't thought of that. Could this be from a mis-match of some sort? | |||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 457 Registered: 9-2001 |
Hmmm ... the band at 1425 would seem to correspond to -OH but the one at 2220 seems to be at a lower wavelength than we'd normally expect. It might be -OH that's shifted. There's also a small band at what might be around 1940 nm, but isn't marked so it's hard to be sure. The small size of that band indicates that if there's moisture, there's very little of it. I think the most telling feature is that the two large bands are relatively narrow, indicating that all those -OH (or whatever gives rise to it) are in similar environments. The most likely cause of that is that they are part of a crystal structure; i.e., a solid. Lois' guess is reasonable. Another possiblilty is that it's a crystalline form of a high-MW alcohol (pentanol or higher, which are solids) or even a high-MW carboxylic acid (octanoic acid and higher acids melt at higher than room temperature); in a pharmaceutical context, salicylic acid seems a reasonable guess. \o/ /_\ | |||
| Tony Davies (td) Moderator Username: td Post Number: 269 Registered: 1-2001 |
Hi Pierre, Fernando, Karl and Lois, I was thinking that the 1425 peak could be a non-hydrogen bonded OH but then worrying about the 2220 peak, Could this be the corresponding OH combination band? Silanol could be correct; do we know anything about Si-H absorbtions? Do you know anything about the source of this spectrum Pierre? Best wishes, Tony | |||
| Lois Weyer (lois_weyer) Senior Member Username: lois_weyer Post Number: 43 Registered: 7-2002 |
Silanol bands, slightly shifted? Plus some water. | |||
| Karl Norris (knnirs) Senior Member Username: knnirs Post Number: 58 Registered: 8-2009 |
Pierre, I have not attempted a search, but I have seen spectra like this as reflection spectra of dry powders. These appear to have a weak water band at 1950 nm. If it is important,I can do a search of spectra I collected more than 30 years ago. Karl | |||
| Fernando Morgado (fmorgado) Senior Member Username: fmorgado Post Number: 28 Registered: 12-2005 |
Hello : Look as some aquous sample, probably the spectra was collect using vials with not standarizated pathlenght. The first peak look as some Ar-OH or O-H, the other maybe O-H or C-H........mmmmmmm I don�t know | |||
| Pierre Dardenne (dardenne) Senior Member Username: dardenne Post Number: 64 Registered: 3-2002 |
Dear All, Does someone recognize the shape of these spectra? Pierre
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