FMF - Friends of Minerals Forum, discussion and message board

[Jim]

Hi John and all,

Indeed, Brazilian amethyst does lighten in color when kept in direct sunlight for prolonged periods. The change may not happen or be noticeable after a week or so, but my sources routinely tell me a different story if the amethyst is exposed for say one month or more.

Rock Currier even talks about this in his recent MR articles.


Cheers,

[David Von Bargen]

Rock also talks about amethyst fading at https://www.mindat.org/forum.php?read,5,115692,116035#msg-116035

Roger Pabin mentions that amethyst from Nebraska will bleach after a couple of days in the sun. So it does seem as if amethyst from some localities can react slower or faster to sunlight.

[Matt_Zukowski]

These discussions are good. John, I think it is good to add data on minerals that are not affected or are improved by exposure to light. You will see in the list mention that Hilton yellow and Elmwood fluorite appear to be light stable and that Morganites from various S. CA pegmatites would be left in the sun to "bring the pink up."

John - can you more specific on what IL fluorites improve with exposure - if so, I'll add it.

Anyone who can critique anything in the list or any way to add to the list is encouraged to do so.

[John S. White]

Sorry Matt, that was something told to me many years ago and I do not know the specific source names.

[Peter Megaw]

The octahedral fluorites from the Navidad Mine in Rodeo Durango are deep grape purple when mined, but the commercial dealers leave them in the sun for 6-9 weeks until they turn an attractive shade of pink.

[Peter]

Fluorite from a over 1m3 pod enclosed in skarn, with fine grained molybdenite mineralization as a rim around (in the skarn) had a deep violet color when collected by me at the 220 m level of Yxsjöberg Cu-Scheelite deposit in Sweden in december 1977. The fluorite would luminesce after hitting it with a sledge hammer, or by shining on it closely by the head light, or at home from heating it in hot water.
After a couple/few years it turned a light sky blue color (stored in garage with no direct sunlight but N facing window), and after additional years it turned entirely white, which almost no geologist or collector could distinguish from quartz (NO, no obvious cleavaged!). Beeing white it had lost all its luminescent properties.

[Matt_Zukowski]

I just got a great book from Amazon: Howie (1992) The Care and Conservation of Geological Material.

This book pulls together a ton of the physics and chemistry of mineral specimen degradation. It is well written. I believe that whatever I was imagining pulling together about this subject has been done before, at least as of the 1992 copyright of this book.

What i will do is, over the next week or two, read the book and update this thread on light sensitive species with the intention of getting the info on the internet without rehashing the book. What the book lacks is locality specific information - that is always welcome - so if you can think of anything please post it and I will accumulate everything and post it here and on mindat.

Please note that Howie, 1992 also includes information on thermal and humidity sensitivity, and includes chapters on the special sensitivity of elements, of sulfides, of sulfosalts, and of marcasite and pyrite. There is a chapter on protecting yourself from mineral specimen dangers as well. I browsed it and am happy to have the book (can't you tell).

[Jordi Fabre]

That's just wonderful! It will be a fantastic job Matt, thanks a lot.

As soon as I have time, I will add new additions to the list.

Jordi

[Matt_Zukowski]

The Howie book is excellent. One of the interesting suggestions he has is that a good test for color stability is to heat a small fragment to 200C for an hour or so and look for changes. No change indicates long-term color stability to light. This test should not be used for yellow sapphire.

[Matt_Zukowski]

Updated list, perhaps needing reorganization:

Anglesite (yellow and brown to colorless)

Anhydrite (blue to colorless)

Apatite group (mauve or pink to colorless)
- Pakistan, Afghanistan* pink fades
- La Marina Mine, Pauna, Boyacá Colombia* pink fades
- Moro Velho Mine, Nova Lima, Minas Gerais pink fades
- Himalaya Mine, CA

Aragonite (w/ color)

Argentite (darkens)

Aurivilliusite

Baryte (colorless or blue to darker; blue to colorless; yellow/brown to green or blue)
- “Hartsel” Baryte can turn from white to blue in sunlight.
- Moscona Mine Baryte goes from white to blue in sunlight but reversible (just one more time)

Beryl v Aquamarine*
- Blue beryl can be made irradiating certain pale natural beryls but like maxime, the electron trap is shallow and so unstable. Fe-colored aquamarines are perfectly stable.
Beryl v Emerald
Beryl v Maxixe* (Blue to colorless or pink)
Beryl v Morganite (apricot or purplish to pink; pink to paler pink)
- One Afghanistan find, pink beryl turned deep yellow with a few hours of sunlight.
- S. CA pegmatites, morganites would be left in the sun to "bring the pink up"

Brazilianite (green to colorless)

Bromargyrite (darkens, Ag liberated)

Calcite (colors fade)
- Elmwood, TN*
- Santa Eulalia (yellow ones from Santa Eulalia temporarily turn pinkish on 15-20 min exposure to sunlight, turn white permanently with 30-60 min exposure to sunlight.

Celestine (blue to colorless)

Chlorargyrite* (gray to violet-brown, Ag liberated)

Cinnabar (red to black metacinnabar)

Corderoite
- McDermitt (Cordero) Mine, NV, Pink Corderoite turns a mouse gray color

Corundum (yellow to colorless)

Crocoite (red to brownish)

Creedite (purple creedites are VERY light sensitive)

Cuprite* (darkens, Cu liberated)

Diamond (yellow to green; red to pink)

Djurleite
- Mount Gabriel, County Cork, Ireland

Fayalite (green to blue)

Feldspar v Amazonite*

Fluorite (pink to colorless; green to purple; blue or purple to colorless or pink)
- Bingham, NM, blue will fade with exposure to sunlight.
- El Hamman, Morocco, Ink blue pales with 30 min direct sun exposure *
- Elmwood fluorite is reported to be stable.
- Haute-Loire, France, sky blue turns colorless with 30 min direct sunlight.
- Hilton yellow fluorite is reported to be stable.
- Navidad Mine, deep grape purple when mined, but miners put in sun for 6-9 weks to turn them pink.
- Sant Marçal, Montseny, Spain, deep blue turns dirty green with 1 hr direct sunlight exposure. *
- Weardale (Cowshill area), Pale green changed to purple almost immediately on exposure to daylight (not even direct sunlight!).
- Weardale (Rogerly, Heights, Cement Quarry, and the old White's Level), green are all potentially unstable, though to varying degrees. Purple color appears more stable. Deep green fluorite from the Rogerly (Solstice Pocket) permanently changed almost instantly to a muddy gray-green if exposed to a LWUV lamp; this process took longer in sunlight.

Gypsum (pink fades)

Halite (blue or yellow may change)
- Huantajayite (argentian halite, contains silver halides)
- Searles lake, pink color from halophylic bacteria and algae fade with exposure to sun.

Haüyne (blue pales)

Hisingerite (red to brown)

Ianthinite (purple to greenish yellow)

Inesite (redish to whitish)

Kleinite (yellow to orange)

Lepidolite (purple to gray)

Marcasite (w/ high humidity - can speed up oxidation)

Metatyuyamunite (yellow to green)

Mercury Halides like Aurivilliusite

Miargyrite (darkens)

Miersite (darkens, Ag liberated)

Mosesite (yellow to green)

Nepheline (pink to colorless)

Orpiment (slow decay)

Pabstite (pink to colorless)

Pararealgar*

Phenakite (red to pink)
- Lemon yellow phenakite from Mt Antero turned colorless after one day in sunshine.
- Orange/brown phenakite from the emerald/alexandrite deposits in the Urals turn colorless or white depending on inclusion content within hours or days if exposed to UV light.

Proustite* (darkens)

Pyrargyrite (darkens)

Pyrite (w/ high humidity, light can speed up oxidation)

Pyrostilpnite (darkens)

Quartz (most colored quartz is light sensitive)
Quartz v Amethyst (fades)
- Brazilian amethyst
- Nebraska amethyst will bleach after a couple of days in the sun.
Quartz v Citrine
Quartz v Morion
Quartz v Rose* (fades)
Quartz v Smoky (smoky to greenish yellow to colorless)
Quartz v Agate
Quartz v Opal

Realgar* (red to yellow pararealgar)
- Realgar is only sensitive to green light

Rutile (pale to darker)

Scapolite (violet to colorless)

Silver, native – can tarnish when exposed to light and moisture
Silver Halides (these generally darken and Ag is liberated)
Silver Sulfides/Sulfosalts like Miargyrite

Sodalite (blue)*
Sodalite v Hackmanite* (red to green, blue, or colorless)

Spinel (red) -> fades

Spodumene v Hiddenite (fades)
Spodumene v Kunzite (pink to colorless)

Stephanite (darkens)

Tetrahedrite (darkens slowly)

Topaz* (brown to colorless or blue; blue to paler or colorless)
- Most Thomas Range, UT sherry topaz xtals turn colorless with exposure to sunlight.
- Some topaz xtals from east side of the Thomas Range, UT start out as sherry but turn pink after one to three weeks in the sun. This is due to an unusually high content of pseudobrokite inclusions. The pink is stable, at least after one year of leaving these in the sun.
- Some topaz from the Little Three Mine were collected as colorless but turned blue upon exposure to the sun. Blue crystals that came out of the 1976 and 1991 pockets became much more blue with exposure. This blue color appears stable.
- Sherry colored topaz from Villa Garcia, Zacatecas, Mexico is reported to have stable color.
- The sherry colored portions of topaz xtals from Mokrusha Mine, Urals fade and seem to turn light blue with exposure to sunlight.
- Volodarsk/Volhynsk, Ukraine topazes usually start our dark orange but fade quickly with exposure to sunlight. Bicolored samples found in some pockets (light pinkish champagne and blue) seem to be more stable (at least for 15 years).

Tourmaline (some pink and red fades)

Tuperssuatsiaite
- Tuperssuatsiaite specimens from Aris started out mauve but turned green.

Vanadinite (red or orange to darker)

Vivianite (green, blue) -> decay*

Wulfenite
- Red Cloud wulfenites will fade over time

Xanthoconite (darkens)

Zircon (brown) -> fades*

[Jordi Fabre]

Great job Matt!

I already revised the list, I added few infos and I cleaned some minor grammatical details. For me now the list is ready. We will wait for a while and if no one send more proposals or corrections, next Monday we will send it to Mindat as well as to the Spanish Forum (translated to Spanish language)

Thanks again Matt.

[John S. White]

I have to say that I am more than a little uncomfortable about this project. It now seems that a very large proportion of the favorite minerals of collectors are included, and my feeling is that this will discourage many of the more timid collectors from buying these minerals. There certainly are varying degrees of risk involved. For example Brazilian amethyst is included yet I have had a piece in full sun for 10 days now and it hasn't changed at all. I have a control piece which was once connected to it so I can easily observe if there has been a change. I will continue to leave it in the sun for at least a month, but my guess is that it will not fade. The same may well be true of many of the other minerals on the list. Some colors can be restored, yet I see no mention of that fact. I have had a faceted yellow sapphire for 30 or more years, it has not faded. The Smithsonian has had a yellow diamond on display for 40 years or more, it has not faded. Same with Elmwood calcites, The old Red Cloud wulfenites at Harvard have not faded. Blue topazes from Zimbabwe have not faded. I am unaware of any tourmalines that have faded. On and on.

This seems an alarmist's list and I think it will do more harm that good. I can see listing those with a high probability of fading or darkening, but not those that are highly questionable, at least not without giving specific examples and reporting the condtions that are alleged to have caused the fading.

Also, opal crazes, I am not sure that it fades. It is not a variety of quartz, by the way. Then there is that old bugaboo selenite. The mineral is gypsum, even if sold as selenite.

[Jordi Fabre]

[Matt_Zukowski]

I think John makes good points (list may be inaccurate or misleading; this may induce paranoia that is detrimental to the hobby). I have been thinking the same thing because putting together this list has made me paranoid about putting a lot of my stuff out on my shelves for display. One thing Howie does provide is four tables:
1) Light-induced color changes (~57 species)
2) Light-induced decomposition (8 species)
3) Light-accelerated surface reactions (~88 species).
4) Recommendations for light protection.

Under table 4, he has two divisions as follows:

A) Light protection usually required
Beryl v Maxixe
Brom(arg)yrite
Chlorargyrite
Cinnabar
Embolite
Ido(arg)yrite
Miersite
Proustite
Pyrargyrite
Realgar
Vivianite

B) Low light level suggested
Amber
Argentite
Beryl v Morganite
Celestite
Chalcocite
Cuprite
Lepidolite
Quartz v rose
Spodumene v Kunzite

With this table 4 I think he is trying to indicate that some minerals you should always worry about (table 4A); some minerals you should be careful with (table 4B), and other minerals may, but less frequently, have problems (not in table 4 but in the other tables). I think, by making these divisions, that Howie is trying to relate some of the complexity of this subject to his reader. But I don't think he has captured it because, among other things, he lacks locality specificity. I also am not sure about Howie's selection of minerals - shouldn't a nice gemmy vanadinite be on one of the table 4 lists?

All this suggests that what is required is a wiki produced by a group of knowledgeable people. Someone tells me that Elmwood calcites are light sensitive, so they ended up on the list. John comes back with a disagreement. A wiki process could proceed to iron out the competing assertions until what is left is what everyone can live with. Only a process that includes much collaboration can produce information that avoids the two problems John cites.

[Jordi Fabre]