When I began to collect mineral specimens in earnest in the autumn of 2009 I was confronted with the dilemma of determining which aspect(s) of the hobby I should explore. As I delved into mineralogy I became most interested in its systems of classification based on chemical composition, as devised by mineralogists Edward S. Dana, Hugo Strunz, and Ernest Nickel. Based on the 9 primary mineral “families” (Native Elements, Sulphides, Halides, Oxides, Carbonates, Borates, Sulphates, Phosphates, and the Silicates) I decided I would collect minerals based on chemical composition or, more specifically, based on each naturally occurring expression of every element found in nature.
I find it fascinating that given certain geological processes an element may occur either homogenously by itself as a “pure form” or combined with various compounds to form a number of mineralogical species that are demonstrative of at least some of the 9 mineral families. I realized I could organize my acquired specimens into elementally themed groups or “suites” from the Periodic Table: a suite for Zinc, a suite for Arsenic, another for Silver, etc. This systematic approach also stems from my own boyhood attempts at obtaining a sample of each element from the periodic table, something which proved to be impractical, if not dangerous, for a twelve year old to do. Although Element Collecting remains a fascinating hobby, as exemplified by notable collector Theodore Gray, and now with on-line sites like Metallium…
Of course not every element naturally forms species identified with each of the mineral families. Lead, for example, can be found in nature in its native elemental state (albeit rarely) as well occurring widely as a sulphide (Galena), a carbonate (Cerussite), and a number of other (less common) halides, oxides, sulphates, phosphates, and silicates. By comparison, an element like Strontium would never be found in a pure native state, its “purest” forms manifesting in nature only as a carbonate (Strontianite), a sulphate (Celestine), a phosphate (Stronadelphite), and apparently as a rare halide with the recently discovered Strontiofluorite. So some elements are more broadly represented more or less across the 9 mineral “families” like Iron or Copper; other elements like Beryllium or Scandium would only total one or two species, while Technetium or Europium would not appear to have any representative species at all.
I choose to define applicable species for each of the naturally occurring elements under the following four main criteria:
All mineral species to be acquired and collected must be approved by the International Mineral Association (IMA). Any species described as “discredited” or “published without approval” will not be considered. Approved specimens are then categorized within the accepted (Nickel-Strunz/Dana) system of mineral families and their sub groupings:
Elements: also includes Alloys/Non-metal Compounds, and Carbides/Nitrides/Silicides
Sulphides: also includes Arsenides, Selenides, Antimonides, Tellurides, Bismuthides
Halides: divided into Fluorides, Chlorides, Bromides, and Iodides
Carbonates: includes Nitrates when applicable
Sulphates: also includes Chromates, Molybdates, Tungstates, and applicable Niobates and Tantalates
Phosphates: also includes Vanadates and Arsenates
Organic Compounds: mostly represented in the Carbon suite, as almost all of the IMA approved species that meet my collecting criteria within this family are mainly hydrocarbons. Guanine and Urea have been omitted as I’d rather not include these common metabolic waste products as specimens.
Each species to be collected are “pure” or “non-alloyed” forms of a given element. Obviously this includes all species classified as Native Elements (Gold, Diamond, Copper, Sulphur, etc.) as well as the other mineralogical families: the pure element combined with sulphur as a sulphide, halogen(s) as a halide, silicon/oxygen groupings as a silicate, and so on. Chalcocite (Cu2S) and Pyrite (FeS2), for example, are acceptable but not Chalcopyrite (CuFeS2) being a sulphide of the two combined elements Copper and Iron. Likewise Calcite (CaCO3) is ideal for my collection, but not Dolomite (CaMg[CO3]2.) Only natural alloys by themselves (including the carbides, nitrides, and silicides) as classified under the Native Element family are acceptable provided the chemical composition incorporates no more than two elements. For example I would acquire Tongbaite, a carbide of Chromium (Cr3C2), but not Cohenite which is a carbide involving Iron, Nickel, and Cobalt ([Fe,Ni,Co]3C.) I assign alloy species to the elemental suite based on the “leading” element in the chemical formula: for instance, Leadamalgam (HgPd2) and Potarite (PdHg) are placed in the Mercury and Palladium suites respectively.
I quickly learned that the chemical formulae among applicable species within the same mineral family or subfamily can be variable due to the inclusion of additional elements/compounds. I noted in many cases that the chemical composition for an applicable species does not rigidly adhere to some basic recipe characteristic of its mineral family. The general formula for a phosphate is XPO4, where “X” is the metal atom(s) bonded to a phosphate group(s); yet many phosphates require the addition of a halogen to be stable. This is the case with Pyromorphite (Pb5[PO4]3Cl), the only naturally occurring pure Lead phosphate. There are some species which show a duality or plurality of mineral family characteristics. For example, Stibnite and Metastibnite (both Sb2S3) are perfect Antimony sulphides for my collection; however I find it necessary to also include Kermesite (Sb2S2O); likewise I must also include Spurrite (Ca5[SiO4]2[CO3]) which is a suitable Calcium silicate that also contains a carbonate.
As a result I have had to establish some “rules” as to which additional elements or compounds may be part of a given formula, and variations thereof, before a species in question no longer meets my criteria for acquiring/collecting.
All non-metal components are permitted under the following provisions:
Hydrogen and/or Oxygen: but not as H2O or OH, see Criterion 3
Carbon: usually as a carbonate or organic compound
Nitrogen: usually as a nitrate or organic compound; usually as an ammine complex as either the main constituent of the mineral, e.g. Salammoniac NH4Cl, or as an additional compound within an applicable “non-alloyed” mineral, e.g. Ammineite CuCl2(NH3)2 (a copper chloride with an ammine complex) but not Brontesite (NH4)3PbCl5 (an ammine and lead chloride)
Phosphorous: usually as a phosphate
Sulphur: usually as a sulphate or sulphite; usually as a sulphide but not when part of an arsenide, antimonide, telluride, or bismuthide
Selenium: usually as a selenide but not when part of an arsenide, antimonide, telluride, or bismuthide; also as a selenate or selenite; considered interchangeable with Sulphur
Fluorine, Chlorine, Bromine, Iodine: usually as halide compounds
In keeping with this criterion of “non-alloyed” forms, only the following metal and semi-metal/metalloid components are permitted under these provisions:
Boron: only as a borate
Silicon: only as a silicate
Vanadium: only as a vanadate
Chromium: only as a chromate
Arsenic: only as an arsenate
Niobium: only as a niobate
Tantalum: only as a tantalate
Molybdenum: only as a molybdate
Tungsten: only as a tungstate
Every mineral that I have identified as suitable for collecting has been checked against the classification systems of Edward S. Dana, Hugo Strunz, and Ernest Nickel, especially if the family designation was not immediately apparent. For example, I would have thought Asisite (Pb7[SiO4]O4Cl2) to be a silicate but it is actually classified as a halide. In the case where a species is classified differently between Dana, Strunz, or Nickel I will usually select the classification assigned by the majority (often with a slight bias towards Dana), sometimes using the addition of Hey’s Mineral Index in determining the final consensus.
All species collected are anhydrous, containing no water or hydroxyls in the chemical formula. This criterion was implemented in order to obtain the “purest” form of a given compound that occurs naturally and to establish another useful boundary of collecting parameters.
All species/specimens are to be naturally occurring (geologically or meteorically.) While I concede that synthetically produced minerals still must obey the natural laws of chemistry and crystallography, I find that there can be no substitute for the beauty of crystals produced naturally due to the serendipitous combination of geological forces, elements, and time. By comparison, so called “man made” crystals appear unattractive due to a complete lack of this natural heritage so as to be excluded from my collection.
I am aware that almost any mineralogical specimen will never be a pure example of a given species; invariably there will always be traces or inclusions of other compounds and elements. The Amethyst variety of Quartz, for example, is distinctly coloured due to inclusions of Manganese, yet the chemical formula is still written as SiO2. I only allow for inclusions or so called impurities provided that they are not present to such an extent as to be reflected in the IMA approved chemical formula.
Some species, while meeting my criteria for collecting, may be omitted due to extreme rarity or difficulties in physically maintaining the specimen(s). Some minerals, like Argentite or Ice, can only exist in temperatures ranges outside of what would be considered practical or otherwise “room temperature.” Other species may be highly hygroscopic; I have noted several of these which meet my criteria for collecting, but may prove impractical to obtain and/or keep. In any case, as my collection grows and I become more experienced, I trust I will learn which species may prove too difficult to obtain and store and make omissions as required, if any. While I will also acquire minerals that may happen to be toxic or radioactive (and observe all precautions) I may decide not include any minerals that are extremely explosive, flammable, or corrosive. Again, this depends on my development of experience and familiarity with potentially dangerous species.
In addition to the above four criteria listed above I also adhere to the following:
No specimen will be accepted into my collection without locality information. I prefer to have as much information as possible regarding the point of origin of my specimens, although this is not always possible. Typically I will not include a specimen with a vague geographical pedigree unless the specimen is exceedingly rare or difficult to obtain, or, at times, if the country named is comparatively small. A label that says only “Sri Lanka” or “Madagascar” is more forgivable than a label that only says “Australia” or “Russia.” Additionally, I do not attribute additional geographical information to my specimen labels, no matter how tempting it may be to do so. Although I am aware of a number of well known localities yielding “classic” specimens of interest to collectors, (Smithsonite from the Kelley Mine of New Mexico, for instance) my own interest in localities tends to be rather secondary to the acquisition of excellent, (or at least adequate) damage-free examples of representative species.
I will not acquire specimens that are damaged. At present I already have a number of specimens in my collection that do show some damage, the majority of which were obtained when I was first interested in mineral collecting as a teenager. Currently I will tolerate damage only as minute chipping at 20X magnification, or if all crystals are complete and undamaged except for the edges where the specimen had been attached to the surrounding matrix. Specimens that have been artificially modified are also not tolerated. As I am fascinated by the natural euhedral forms of crystals I would refuse any specimen that has been tumbled or polished. I find it acceptable to clean or otherwise prepare a specimen to improve the visibility or accessibility of its crystal(s), such as trimming excessive matrix or using appropriate acids to safely dissolve any obscuring carbonate material.
I prefer, whenever possible, to have at least one specimen representing each habit that a given species may naturally assume. It would be my goal in the case of Fluorite, for example, to acquire a number of specimens each representing at least one of its habits: cubic, octahedral, botryoidal, etc. Availability in colour variety is also taken into consideration, which further expands the varietal possibilities for a given species. Naturally I seek to obtain specimens that show clear euhedral form whenever possible. Specimens exhibiting pseudomorphic or twinned forms are also welcome, but I prefer to obtain the “textbook standard” of a given crystal first. I also require a specimen to be comprised of only one species, although this may not always be possible.
While I continue to consult field guides and various books written on mineralogy I have found the websites www.mindat.org and www.webmineral.com to be extremely useful in looking up a species’ chemical composition, IMA status, and morphology. While these sites may not have the most accurate locality information they are continually updated, particularily when a new species has been discovered or declared valid.