By Bill Shelton
Eudialyte is a proper mineral species and the name of a mineral group. There are 26 species within the group; I find about 20 are very restricted in their occurrence meaning they are present at only one or two localities. The chemistry and structure of most of the species have only been studied in detail in the last 10 to 15 years. Variability in the chemistry was noted over 100 years ago and this is one reason why we now have so many species. In some ways, this is reminiscent of the similar circumstances surrounding the tourmaline group. Fortunately, the name of that group (tourmaline) is not currently accepted as a mineral species. As a collector, I am interested in similarities and differences between species within a mineral group. The locality data is also significant to me; for the eudialyte group many species occur within the former Soviet Union which is my main collecting focus.
Based on webmineral and mindat, I have found the dates for all 26 species in the eudialyte group. They are, indeed, a new group because all of the species were identified since 1990 with one exception. Eduialyte dates to 1801 and has been accepted for a very long time. The next entry is in 1990, and that would be alluaivite. Three more were accepted in 1998: they are kentbrooksite, manganokhomyakovite and oneillite. In 1999, we add khomyakovite. All the rest are in the 21st century. So, 20 species might be called relatively new. Few groups can claim this status.
Color may be one of the most interesting properties among minerals; many collectors value it above most any other trait. I decided to try and create a list of all the colors suggested for eudialyte, the species [as presented in a number of different sources]. They are red, pink, rose-red, carmine red, cherry red, orange-red, brownish-red, orange, yellow, yellow-brown, greenish-yellow, green, violet and brown. When I collected at Mt. St. Hilaire, there were red, pink and orange crystals which we assumed to be eudialyte. Only occasional eudialyte group members and then perhaps only certain examples of other group members exhibit colors not already listed for eudialyte the species. I find color to be unreliable and probably misleading in the identification of the 26 species included in this group.
The proper identification of any eudialyte group member will be a complex process. As the scientific community moves toward structural mineralogy and site occupancy for species, the mineral collector is somewhat left in the lurch. One good aspect (if you like more species) is this: new species identifications based on structural differences have resulted in many new species being described. In 1990, there were two species in this group; by the year 2000 we had six. Currently, the total is up to 26 (or more) and that is worth factoring into your collecting plans. Identifying the 26 members is a challenge; the following article will briefly describe the members and their chemistry as well as noting some important locality data. You probably need X-ray data, site occupancy information and detailed chemical analysis to positively identify a given species in the eudialyte group.
The chemistry of eudialyte is in a word, scary or so it seems to me. Is there a mineral group that contains about half of the elements of the periodic table besides the eudialyte group? One reference, Khomyakov (2008) claims this to be a fact. I can confirm at least a third of all elements are at least occasionally present in the 26 group members. My claim is based on published analytical data from multiple sources but is not to be considered as exhaustive or necessarily complete. The difference is possibly composed partly of additional REE’s that I have not found in the references searched so far. Based on the ideal formula (see Back, 2014), you might assume that it would be easy to separate the 26 species; I’m not so sure it will be a simple matter. In the ideal formulas as given in Back (2014) there are 18 elements plus REE and vacancy. The total chemistry is even more complex; see table 1 below.
Table #1 Elements in eudialyte group minerals.
Al Ba C Ca Ce Cl Dy Er F Fe Gd H
Hf K La Mn Na Nb Nd O P Pr S Si
Sm Sn Sr Ta Ti W Y Yb Zr
Rastsvetaeva et al (2012) refers to “isomorphic substitutions at several structural sites”, p. 496. The descriptive notes for the sites are non-equivalent in various sources but the version below is hopefully correct. The N sites (there are 5) usually host Na; K, Sr, Ba, Mn, Ca, REE, Y, H3O, H2O, Ce and vacancy are also noted as possible occupants. The four M sites may contain (M1) Ca, Mn, Na, Sr, Fe and REE. (M2) may house Fe, Mn, Na, Zr, Ta, Ti, K, Ba and H2O. (M3 and 4) may contain Si, S, Nb, Ti, W, Na, Ce and vacancy. The Z site houses Zr, Ti, Nb, Al, Fe, Mn, W and vacancy. The O’ site contains O, OH, and H2O. The “Si” site contains Si and Al. The X (1 and2) sites contain Cl, F, H2O, OH, CO3, SO4, AlO4, and MnO4. Jonhsen et al (2003) states that “a wide variation in chemical composition” (p. 786) is present in the eudialyte group minerals; further he claims “the theoretical number of mineral species based on the non-silicate cations only, extends far beyond several thousands”, (p. 788). This may turn out to be a much more difficult project than we have time to resolve. The general formula would look like N 1-5, M 1-4, Z, Si, O’, and X but see Rocks and Minerals, Vol. 89 (2014), Rastvetaeva (2012) or Johnsen et al (2003) for more detailed information. Also, see the site data example under the description for kentbrooksite.
Solid solutions, either complete or partial, are widespread in the eudialyte group minerals. Examine and compare the ideal formula for zirsilite-(Ce) and carbokentbrooksite – you will find (Na,Ce) in carbokentbrooksite while zirsilite-(Ce) has (Ce,Na). This is typical of solid solution minerals and shows relationships fairly clearly. The names kentbrooksite and ferrokentbrooksite are considered to be the Mn and Fe analogues but you will notice another difference. Some REE is present – but only in the ideal version of kentbrooksite. We can speculate that a series may extend from REE bearing to REE free examples in addition to the Mn-Fe series. Alluaivite, rich in Ti, suggests a partial series might extend toward some Zr rich (the usual state) eudialyte species. At least a few possible new species might exist at least theoretically. Khomyakovite, richer in iron, and mangankhomyakovite, richer in manganese, offer another example of a series. Via multiple substitutions, several species can be considered as members of solid solutions, also called solid solution series. Just by manipulating the Fe and Mn, in ideal formula terms, there are possibly a dozen more new species. Occasionally, we find a eudialyte described in terms of percentages of various species. For example, Chakrabarty et al (2012) describes hydrothermal eudialyte from Sushina as “representative of the solid solution series between kentbrooksite- taseqite-Ce-zirsilite”. Rastsvetaeva et al (2012) states “the relative amounts of the alluaivite, eudialyte and kentbrooksite structures in rastsvetaevite are about 50, 40 and 10 % respectively” on page 495. This is another way to describe a specimen. As a collector, I would like to be able to label the sample as a species in common usage today.
One of the slabbed specimens that is in my collection is from Khibiny; according to RAMAN data, it is ferrokentbrooksite. This is very interesting to me and I hope the results are accurately portraying the sample. Associated species are lorenzenite, nepheline and aegirine. The label claims it is from the Partomdor mine; maybe actually the Partomchorr Mt.? The specimen is a fair match to the description given by Yakovenchuk et al. in Khibiny (2005) on page 77. Arem indicates “faceted gems well under one carat in size have been cut from Quebec material. They are deep red and extremely rare.” This material is possibly from Kipawa, Quebec; however Arem indicates Mt. St. Hilaire is a source for facetable material. I have seen cabochons and lapidary slices of eudialyte with various minerals such as lorenzenite, nepheline, aegirine and agrellite etc. from Canada and Russia, the spheres and eggs made from this material are all quite interesting and occasionally spectacular. If you decide to buy any of this material for specimens or lapidary use, find out what it might look like and where it is said to come from. Use the internet to help determine if photos are available and then compare the appearance and stated locality. Choose your dealer wisely since you will probably be relying on his expertise and supply chain to be reasonably sure that you get what you expect. As always, a fine crystal is preferable to a grain or mass if you have the choice. Not all species will be available as nice crystals.
Finally, a note about rare earth elements: defined in different ways, I will use the list from Wikipedia. There are 17 REE’s: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Many have been documented within various eudialyte group minerals. Refer to Back (2014) and you will see several members of this group have REE; several more have Ce (cerium) in the ideal formula too. At least nine species fall within this category based on the criteria given above.
In summary, a collector faces some difficult choices when purchasing eudialyte group minerals. From my perspective, I see many more or less massive examples that may or may not contain variable percentages of the rarer species. Of course, a fine crystal is always a better specimen but will you lay out, say $250 or more for a sample that will be partially consumed in a more or less definitive study that may cost several hundred dollars more? Probably, one needs chemical and structural data to be near positive of the correct identity of a given sample. That sounds to me like it will be quite expensive and perhaps not worth your efforts. As noted repeatedly in my brief species descriptions, there is very complex chemistry and variable amounts of components. If one or more elements start to rise in terms of weight percent, the possibility exists that you have a different species; perhaps even a new species! The samples themselves will often be intergrowths with two or perhaps more species present and intimately associated. This is characteristic for this group of minerals.