LIVE REPORT! Tucson 2019 - last updated 2 minutes ago. Click here to watch.
Log InRegister
Home PageAbout MindatThe Mindat ManualHistory of MindatCopyright StatusWho We AreContact UsAdvertise on Mindat
Donate to MindatCorporate SponsorshipSponsor a PageSponsored PagesMindat AdvertisersAdvertise on Mindat
Learning CenterWhat is a mineral?The most common minerals on earthInformation for EducatorsMindat Articles
Minerals by PropertiesMinerals by ChemistryAdvanced Locality SearchRandom MineralRandom LocalitySearch by minIDLocalities Near MeSearch ArticlesSearch GlossaryMore Search Options
Search For:
Mineral Name:
Locality Name:
Keyword(s):
 
The Mindat ManualAdd a New PhotoRate PhotosLocality Edit ReportCoordinate Completion ReportAdd Glossary Item
Mining CompaniesStatisticsThe ElementsUsersBooks & MagazinesMineral MuseumsMineral Shows & EventsThe Mindat DirectoryDevice Settings
Photo SearchPhoto GalleriesNew Photos TodayNew Photos YesterdayMembers' Photo GalleriesPast Photo of the Day Gallery

The Mines and Minerals of Lavrion - Agardite

Last Updated: 31st Jan 2019

By Branko Rieck

The following article gives some insights and experiences the author has made with "agardite". There will be seperate articles about the individual minerals in this group.

The minerals of the mixite group in general and the agardite subgroup in particular usually form thin, elongated (along the c-axis) crystals that are most commonly aggregated to radial aggregates. It is very rare that the hexagonal symmetry can be observed visually without the use of a SEM. Depending on their thickness their color may range from white to light blue, light green or blue-green combinations.

Agardite-(La)


The mixite group follows the general formula:

ACu6(XO4)3(OH)6·3H2O where A = Al, Ca, Y, La, Ce, Nd, Pb and Bi and X = As, P

In compounds where A is divalent, (XO4)3 becomes (XO4)2(XO3OH)

The elements at the A position have an effective ionic radius of ~1 Å (Y: 0.90 to Pb: 1.19 with Al: 0.54 being the exception)
While until now no (PO4)-dominant member of this group has been found in Lavrion, all known (AsO4)-dominant members are reported to occur and one – agardite-(Nd) (Pekov et al., 2011) – even has its type locality in the Lavrion Mining District.

Agardite-(Y)


Table 1: (AsO4)-dominant members of the mixite group known to occur at Lavrion and their theoretical end-member chemical composition. (Note: Agardite-(Gd) not listed here is being prepared for approval)

Agardite-(Ce), CeCu6(AsO4)3(OH)6·3H2O
Agardite-(La), LaCu6(AsO4)3(OH)6·3H2O
Agardite-(Nd), NdCu6(AsO4)3(OH)6·3H2O
Agardite-(Y), YCu6(AsO4)3(OH)6·3H2O
Goudeyite, AlCu6(AsO4)3(OH)6·3H2O
Mixite, BiCu6(AsO4)3(OH)6·3H2O
Plumboagardite, PbCu6(AsO4)2(AsO3OH)(OH)6·3H2O
Zálesíite, CaCu6(AsO4)2(AsO3OH)(OH)6·3H2O

The rare earths are generally trivalent elements, but a few have other valences. Cerium, praseodymium, and terbium can be tetravalent while samarium, europium and ytterbium often prefer the divalent state.

Table 2: Effective Ionic Radii of selected elements (Shannon, 1976)

Name NumberSymbol2+3+
Aluminum13Al0.54
Calcium20Ca1.00
Scandium21Sc0.75
Yttrium39Y0.90
Lanthanum57La1.03
Cerium58Ce1.01
Praseodymium59Pr0.99
Neodymium60Nd1.290.98
Promethium61Pm0.97
Samarium62Sm1.220.96
Europium63Eu1.170.95
Gadolinium64Gd0.94
Terbium65Tb0.92
Dysprosium66Dy1.070.91
Holmium67Ho0.90
Erbium68Er0.89
Thulium69Tm1.030.88
Ytterbium70Yb1.020.87
Lutetium71Lu0.86
Lead82Pb1.19
Bismuth83Bi1.03


Chemical analysis of literally hundreds of samples has shown that there exist solid solutions between all members of this group occurring at Lavrion. This makes the identification of these minerals quite a task.

To give an example:
(Ca0.44, Al0.21, Ce0.16, Gd0.08, La0.06, Nd0.02, REE0.03)
1. Ce is assumed to be trivalent
2. REE used above is the sum of all other REE plus Y
3. The sum of all REE plus Y is 0.35, so it is not a REE dominant mineral
4. The sum of all trivalent elements is 0.56, so the trivalent elements are the majority
5. Of the trivalent elements the sum of REE > Al, but no single rare earth element is greater than Al
6. The divalent Ca is by far the largest single constituent.

The "dominant constituent rule" (Hatert & Burke, 2008: http://nrmima.nrm.se/Dominant%20constituent%20rule.pdf) makes this the mineral goudeyite, because of 4. (trivalent dominates) and 5. (Al is the dominant trivalent element).

One major drawback of the agardite minerals (with the exception of agardite-(Y)) is that crystals tend to show heavy zoning along both the a-axis and the c-axis. This means, that individual needles may be composed of three, four or even more different minerals.

Looking at the mixite-group minerals in general and the agardite subgroup in particular there are a few things the avid collector should be aware of:
1. The color of these minerals depends more on thickness than on chemistry. It is impossible to distinguish the minerals of the mixite group by color.
2. A precise chemical analysis is necessary to attribute a sample to certain species, taking the "dominant constituent rule" into account.
3. SEM-EDX is a method that gives a clue in this respect, and helps to distinguish the minerals of the agardite subgroup from the other mixite type minerals, but usually is not precise enough to really differentiate between actually occurring compositions, especially if the thickness of the crystals is below the beam diameter.
4. EMPA, ICP-OES, LA-ICP-MS or INAA is your method of choice, but admittedly difficult for collectors to get.
5. If a sample is without or insufficient (SEM-EDX) analysis it should be labeled as "mixite group", or, if the dominant REE-content is manifest, "agardite subgroup".

Acknowledgements
Thanks go to Dr. Uwe Kolitsch for constructive comments and diligent proofreading to improve this article.
I also want to thank A. Wagner for preparation of most of the analyzed samples.

References

Hatert F. & Burke E.A.J. (2008) The IMA-CNMNC Dominant-Constituent Rule Revisited and Extended. The Canadian Mineralogist 46: 717-728.

Pekov, I.V., Chukanov, N.V., Zadov, A.E., Voudouris, P., Magganas, A., Katerinopoulos, A. (2011) Agardite-(Nd), NdCu6(AsO4)3(OH)6•3H2O from the Hilarion Mine, Lavrion, Greece: mineral description and chemical relations with other members of the agardite-zálesíite solid-solution system. Journal of Geosciences: 56: 249-255.

Shannon R. D. (1976). Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr A. 32: 751–767.




Article has been viewed at least 825 times.

Comments

In order to leave comments to this article, you must be registered
Mineral and/or Locality  
Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization. Public Relations by Blytheweigh.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2019, except where stated. Mindat.org relies on the contributions of thousands of members and supporters.
Privacy Policy - Terms & Conditions - Contact Us Current server date and time: February 15, 2019 21:54:47
Go to top of page