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Mineralogical ClassificationWhy is Quartz not a Group?
31st Aug 2012 17:07 UTCOwen Lewis
So I ask, why is Quartz not considered to be a Group? Would it not help rationalise the Group/Species/Varieties nomenclature if it was?
31st Aug 2012 17:38 UTCDavid Von Bargen Manager
31st Aug 2012 18:42 UTCOwen Lewis
31st Aug 2012 19:36 UTCAlfredo Petrov Manager
Chalcedony isn't a "group" in the mineralogical sense either, just a variety of quartz, although chalcedony itself has many subvarieties.
One could, I suppose, create a "Quartz group" by adding the species with similar structure, "stuffed quartzes", like Virgilite.
31st Aug 2012 22:15 UTCOwen Lewis
You are, of course, quite right but I still can't find the logic. Cryptocrystalline Quartz is treated as a species of which there are many varieties. But mineralogy does not recognise the concept of 'varieties' in the nomenclature - and yet Mindat is littered with the use of the term and varietal listings...... That this is so indicates that avid mineralogists find the construct of 'varieties' useful (necessary) in differentiation of one mineral from another, closely related. Something needs to give, surely?
David says, in perfect logic, that for a group to exist, it must contain at least two minerals (i.e. two inorganic compounds with differing molecular composition). But then, with only a two level structure for the nomenclature (group/species) and with mineralogists resorting 'under the blanket' to the additional term of 'variety because of the additional level of differentiation is found very useful.
With no IMA recognition in the nomenclature of 'variety', how do we end up, here in Mindat with a nomenclatural structure for the Feldspar group of just two plagioclase species, Albite and Anorthite, with Andesine, Labradorite etc now being relagated as (e.g.) Anorthite var.Labradorite etc.?
It the concept of varietal distinction within a specie is useful - and I do think it is - it is not time to to review and formalise, root and branch, the nomenclature to embrace the concept of 'variety' as the tertiaty and lowest level of nomenclatural differentiation?
This, at the end of this short but elliptic flight, returns me to the concerns of Quartz, a species of enormous variety and already with an unapproved but widely used breakaway species (or group?), that of the cryptocrystalline Quartzes or Chalcedony? The logic of a two-tier only nomenclature is breaking down (has broken down?) being now only honoured in the breach. Is it not time for the formalisation of what has largely already taken place and, along the way, to bring better order to the nomenclature of the huge group of Quartz varieties in the process?
31st Aug 2012 23:32 UTCRalph S Bottrill 🌟 Manager
It's a bit idiosyncratic to have a separate page for chalcedony, it's just because it has so many varieties and it's usually easy to distinguish. We could have separate pages for crypto crystalline calcite, goethite, hematite, magnesite but these don't have so many varieties.
Mineralogical classification works on the basis of defining the fundamental composition limits and crystal structure of minerals. With varieties there is rarely a consensus as to how they are defined, eg gemmologists may think they know what emerald and ruby are but there are no definite rules to say how much Cr or V is required, so no analysis can prove it is or isn't; if the colour looks right the name fits. Some varieties just have fine to microscopic impurities., eg aventurine. Mineralogists recognize that varieties can be interesting and provide useful geological and mineralogical information. but don't feel the need for a new varietal name for every chemical or colour variant of a species. Naming all the colour variants of eg. wulfenite, pyromorphite, mimetite, adamite, etc would just be annoying for the most part. Let's face it it's beyond most of us to remember the definitions of more than a few hundred mineral species, let alone all the named varieties. Most mineralogist would be happy to see most varietal names disappear, except for major gemmological terms maybe. Even chemical modifiers are controversial: names like cupro-adamite are now rejected and and cuprian adamite is frowned upon as it implies chemical information which may be incorrect, and even calling something a copper-bearing adamite is ambiguous: how much do you need?.
The plagioclase group used to have lots of members (oligoclase, andesine etc) but strictly now it is just a series between albite and anorthite. The IMA are working on the thorny issue of what is a series/group/subgroup etc.
31st Aug 2012 23:50 UTCAmir C. Akhavan Expert
Minerals are classified/defined by a specific limited set of properties, basically chemical composition and structure (the way atoms are arranged in the crystal).
And these mineral definitions (sets of properties) are mutually exclusive: one mineral cannot be another, even if that means that a single crystal can contain different minerals (like tourmaline group minerals).
Varieties are defined by properties that specimen have in addition to the set of properties that determines how they are classified as minerals.
There's no limit on the number or type of properties. Color, shape, feel, chemistry, everything goes.
Varietal definitions are not mutually exclusive: a rose quartz can also be a smoky quartz, nothing wrong about that.
Because of that, you cannot expect the same rigor in varietal classification schemes as in mineral classification schemes, and there's no inherent reason why varieties should be ordered in any hierarchical manner (would not work, anyway, as they are not mutually exclusive), let alone why such a scheme should work for all minerals.
And it is actually a good thing that these things are not treated as strictly as mineral classifications, because that way definitions can be changed, improved, fixed, as our knowledge grows, without the fear that we have to overthrow an old classification scheme that did not take into account new property X.
When it comes to quartz and how to sort things, I try to cut down the number of varieties as much as possible. And the first step is to use a (virtual) hammer and smash a specimen and then ask: can you still tell what it was from the shards you got: If yes - variety, if no, not a variety.
Which is to say: habit, growth forms and all these things usually don't count. Provenance definitely doesn't count.
A Herkimer diamond, Selvino diamond, Lake County diamond, Mirabeau diamond, all are simply "rock crystal". "Scepter quartz" is not a variety.
Color, diapheny, inclusions, microstructure, etc. cannot be broken with a hammer:
"Prasiolite", "eisenkiesel", "macromosaic quartz" are varieties.
This is my private system, of course.
Micro/cryptocrystalline quartz is very special and I understand that some mineralogy textbooks treat chalcedony and quartz separately. It makes sense to treat agate as a variety of chalcedony, because that stresses its nature as an aggregate of microscopic and submicroscopic crystals. There's no single crystal of quartz that can be called agate or any other chalcedony variety, the aggregate structure is part of the definition. (and yes, I did deliberately not talk about moganite).
Unfortunately, the term "chalcedony" is used very broadly. It has several meanings and the most narrow and best-defined definitions are those of the two types "length-slow chalcedony" a.k.a. "quartzine" and "length-fast chalcedony" a.k.a. "chalcedony strictu sensu". These two terms are of some importance in scientific literature but much less so among collectors.
If I'd start to subdivide chalcedony, you'd be surprised how little would be left, maybe 5-10 basic types, only 3 types of agate, something like that. And if someone said, "I have this thing labaled 'rudy the rednose reindeer agate' what is it?", I'd pick from these and say "it is just type 4, wall-lining agate made of spherulitic length-fast chalcedony, very common".
Of course, such a system would not be very popular among agate dealers :-D
1st Sep 2012 00:37 UTCRalph S Bottrill 🌟 Manager
The point about habits and growth forms is an interesting one though, it will be interesting to see what others think. I can see your point, eg a pyrite is a pyrite whether it's in a cube or pyritohedron, but if it's in a ball or "sun" I suspect most people would call it a variety? Ditto faden and scepter quartz, maybe Japan-law twins, etc? As you point out varieties are defined indiscriminately, in many ways calling something a barrell-shaped, arsenic-rich brown pyromorphite is is bit wordy compared with "campylite" but at least it's got some definition to it.
Interesting one for discussion!
1st Sep 2012 01:16 UTCOwen Lewis
1st Sep 2012 01:47 UTCPavel Kartashov Manager
1st Sep 2012 10:58 UTCPeter Nancarrow 🌟 Expert
On the other hand, phases which have identical chemistry but distinctly different structure (for whatever physical reason) are classified as separate species which are not members of a group. (e.g. quartz, cristobalite, tridymite etc).
Distinct and commonly occuring variations in the physical appearance of a given species of a particular composition and structure give rise to the variety concept. These may be due to differences such as trace/minor element variations giving rise to distinct colours (e.g. amethyst, rose quartz), or remarkable clarity (rock crystal), growth habit (Faden quartz), inclusion population (aventurine) etc. However, those are all members of one species (in this case SiO2 crystallised in the quartz structure), and multiple varieties of one species do not in themselves constitute a mineral group.
Whilst I do understand the many reasons why the use of variety names is discouraged in formal mineralogical nomenclature and not recognised by the IMA (subjective definitions, proliferation of multiple superfluous, and sometimes misleading, names for the same "variety" from different localities - e.g. rock crystal called "Herkimer Diamond"!, increasingly subtle differences described as new varieties, or abritrary names coined simply as a marketing tool, etc), I would argue that when used with restraint and common sense, the concept of varieties can be a valuable and (mostly!) unambiguous form of mineralogical shorthand. For example, it is much less laborious and more succinct to write "selenite" rather than "macroscopic transparent single crystals of gypsum", or "ruby" rather than "gem quality red corundum coulured by a trace quantity of chromium"!
Adressing another point Owen made above:
With no IMA recognition in the nomenclature of 'variety', how do we end up, here in Mindat with a nomenclatural structure for the Feldspar group of just two plagioclase species, Albite and Anorthite, with Andesine, Labradorite etc now being relagated as (e.g.) Anorthite var.Labradorite etc.?
This is an example of where strict application of the modern definition of "Mineral Species" has led to the demise of formerly recognised "species" from the mineralogical zoo. In the case of the plagioclase feldspars, there is a more or less continuous compositional series from albite at the Na-rich end to anorthite at the Ca-rich end, and the boundaries between those formerly recognised intermediate plagioclase names were completely arbitrary pigeonholes defined at percentage increments of Ab/Ab ratios, and did not really constitute distinct species, so they are no longer officially recognised as such.
However, they have been retained as informal variety names partly because there are so many references in the literature to those old plagioclase series members (and also very many specimens described/labelled with them!), so they are frequent search targets. They are also used in the definition of many rock names, so obliterating them from sources such as the Mindat database would be to remove a valuable source of historical and petrological reference. Relegating them to informal "variety" status is an appropriate compromise.
Pete N.
1st Sep 2012 15:15 UTCOwen Lewis
Peter points out exactly, I think, the reasons why varietal names are (often are) useful and could be made coherent and uniform if authorised and regulated by a suitable central and international authority - namely the IMA. De facto, this process seems to be starting with the logical and sensible removal of several specie names within the plagioclase series (and so these then become what??). These names will continue in use - because it is found useful so to do. This and other pressures seem to me now to have the water spilling over the top of the dam that, to this point, has withheld IMA recognition of 'variety' as a tertiary tier in the nomenclature. The science is served well enough in the definition of group and species; the rules for differentiation within a species can afford to be - must be to be useful - a little more relaxed. This issue is not unique to mineralogy but seems common to other scientific nomenclatures.
The use of trade names or place names as varietal names is surely to be discouraged but it is now impossible, I think, to eliminate entirely. However, a governing body could rationalise what now exists, removing some dubiety along the way. It could (at last) recognise and approve time honoured, important and useful trade names, such as Ruby and Sapphire as distinct gem varieties of Corundum. Where locality names are key to a varietal name, such as Herkimer Diamond or Tanzanite, it seems to me that the rule should be that the name must 'belong' to the differentiating features within that species to which the name was first applied; the locality should lose all proprietary claim to the varietal name.
As Peter points out again, it is counter-productive to allow endless and ever finer differentiation within a species. Distinctive visual characteristics should be the guidelline most useful to the greatest number. With differentiation by colour it should end with ROYGBIV plus colourless,white and black. Where there is strong reason to do so, some elemental rider (e.g. Cuprian Tourmaline might serve to differentiate, say, what is now Paraiba from Blue Tourmaline). In addition to colour there is also simple differentiation by some definable pattern of physical characteristics. Phantom Quartz and Herkimer Diamond are both cases in point. It follows that new(ish) and blatantly commercially driven 'me-tooist' varietal names, such as Merelani Mint (a green Garnet form Tanzania across the border from the Kenyan Tsavorite) and Zultanite should not be included. If others think that there is reason to base varietal differentiation on anything more than either primary colour (possibly with an occasional elemental rider and white and black) or physical features, I would be glad to learn of them.
Commerce may use whatever names it likes (and can persuade a gullible public to swallow). But if proper control is taken over the scientific nomenclature - including varietal descriptions - then outright misrepresentation and fraud become that much harder and with 'the metal meeting the meat' more frequently and with greater success. Judgements become simpler to make and, perhaps, with more frequently satisfactory outcomes. In turn, one sees this as occasioning greater care in the marketing of minerals (and gems in particular) and with sensible restraints on the more exuberant and none too scrupulous marketing practices.
Pavel reminds us that Quartz is, de facto non de jure, a group. So we return to the title question of why is Quartz not a group name in the nomenclature? If it recognised so, it seems to me that good would flow from that. There is clear species differentiation between Rock Crystal (or whatever else one must call large lattice Quartz) and cryptocrystalline Quartz/Chalcedony. The wide and useful list of Quartz varieties could be properly defined and the application of the approved names regulated. The unapproved, 'wannabe' varietal names could be driven into the wilderness to perish or survive at the margin, as trade names without standing.
Just for the Quartz group alone, I think this would be a useful measure, the benefit of which would be felt beyond the halls of mineralogy. Were the general principle established right across the mineral nomenclature, that varietal names are to be regulated and included as a tertiary level of differentiation within the nomenclature, it seems to me that this would not only improve order and clarity in mineralogy's own house but would perform a service of benefit to the community at large.
1st Sep 2012 16:32 UTCAmir C. Akhavan Expert
Varietal names are useful but they cannot "be made coherent and uniform if authorised and regulated by a suitable central and international authority - namely the IMA".
The difference is that the classification scheme for minerals is predefined before the individual mineral is classified.
Varieties are defined post hoc and also not by a predefined set of properties, and that is so for good reasons. These approaches are incompatible and they must remain separate. To say that they should be primarily classified by some criteria like colour would be a fairly dogmatic approach. "Anything physical" makes much more sense, of course, but we do not need "strong reasons" for this.
@Pavel
Do you have a literature or IMA reference for a "quartz group" (not Strunz classification quartz group, that is).
To put berlinite and quartz into one group seems to violate rule 5 as found in
Mills, Hatert, Nickel, Ferraris (2009) The standardisation of mineral group hierarchies: application to recent nomenclature proposals. European Journal of Mineralogy, Vol. 21, 1073-1080.
"5. A mineral group consists of two or more minerals with the same or essentially the same structure and composed of chemically similar elements (see above)."
Of course, one could simply state that Al and P behave similarly "enough" on a crystal-chemical level.
"Similar" is such a nice and flexible term.
It would actually be cool if they did behave the same way, then we might get a solid solution series "berlinite -- pink quartz -- quartz" :-D.
Just kidding, of course.
1st Sep 2012 19:59 UTCRob Woodside 🌟 Manager
The pros are really concerned with the proliferation of names and many of the IMA decisions have been to try and curb the flood as well as improve knowledge. When the microprobe first appeared micron blobs with different chemistry got names, so structure became a necessity. Coal fires and slags were were also eliminated to stem the flood. A paper entitled "Bravoite: A New Member of the Pyrite Group" is more impressive than "Notes on a Nickel Rich Pyrite". (Thankfully Bravoite has been discredited.) Still the flood continues, with more to honour, more to deal, and more to collect. It is both good and bad. I'm grateful for the wonderful new knowledge, but I have a hard time knowing what people are talking about with all the new names.
The move to introduce groups was also an attempt to deal with this, but it seems to have backfired. With groups in place the answer to "What is that?" becomes, "It is in the such and such group but has...". There was an opportunity to reset the hierarchy so that species occurred at what is now considered the group level and the current species as the disdained varieties. One can usually identify the current groups by sight, but the chemistry and structure are beyond my eyes. The solution as I see it is to collect at the current group level.
2nd Sep 2012 00:25 UTCPavel Kartashov Manager
look http://www.mindat.org/min-4395.html and will found Weinebeneite in Zeolite group. ;-)
2nd Sep 2012 00:33 UTCRob Woodside 🌟 Manager
4th Sep 2012 13:25 UTCOwen Lewis
-------------------------------------------------------
> ....In spite of the 7 page
> check list of all the properties that the IMA uses
> for considering a new mineral, it is really only
> chemistry and structure that define new species or
> groups. This is a long way from the curiosity
> cabinets of the 18th century where visual
> appearance and macroscopic properties sorted out
> things. Collectors and field geologists rely on
> visual differences and simple macroscopic
> properties to sort things out and decide if the
> new expensive techniques should be invoked. The
> pros themselves often rely on visual appearance
> under a scope to decide what to analyse.
Amen to that Rob. There are two issues, I think. The first is the concern you describe about the level of detail at which it is useful to differentiate between species (at the risk of ever creating new ones in a flat naming structure). The second is the extent to which identification of varieties within a species is useful and can be controlled to prevent frivolous, misleading or simply unhelpful use. I see a useful link in addressing these two issues together.
It seems to me that a declination officially to define and list varieties within the mineral species is short sighted. Whatever policy may be, mineralogists - and all others who need to rely on the IMA approved nomenclature, continue to rely quite extensively on varietal labelling and, given current IMA thinking and practice, this need seems set to increase rather than decrease in the future. For example, there is the practice now of listing only two species of plagioclase Feldspar, the end members of the solid solution series Albite-Anorthite. So be it - but the old names under which the series has been divided for maybe a hundred years remain useful to most and thus Mindat - and no doubt elsewhere as well - becomes littered with entries such as 'Anorthite (var Labradorite)'. But, to follow the logic of those who say that varietal names must remain without fixed definition and formal recognition, Anorthite (var Labradorite) might just as properly be listed as 'Anorthite (Var Rob's Rainbow)? I think not. It can't be argued sensibly that uncontrolled proliferation of varietal names is useful or other than a disservice to those who need to be exact in what they are describing in whatever walk of life they may be. Saying that fixing and formalising a useful list of varietal names 'can't be done' may be an opinion but is surely not a fact. If the will to do this is sufficient, then it can and will be done. IMHO, it should be done.
Taxonomic structures are not only a scientific necessity but are a social necessity too and serve the common good. There is no international body other then the IMA that is placed to devise and regulate the taxonomy of minerals down to and including approved varietal distinctions. If it continues to decline to do so, that will be a shame.
To return to Quartz. There does seem to be merit in elevating Quartz to group status. Taken part and parcel with adoption of 'variety' as the tertiary level in the taxonomy of mineralogy this would also bring order and orthogonality to a situation that presently remains in some disorder, with mineralogists not only having to rely on unapproved use of varietal labelling but of significant sub-varietal labelling as well! If there is a substantial argument for continuing with this present arrangement, it would be good to hear it.
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Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: May 10, 2024 02:54:42