A slight cringe at a Yellowstone National Park sign

I've been back from my month long trip to South Dakota & surrounding areas for a couple of weeks now.  There's lots of great geology to talk about, so a lot more will come as I get around to it.  But for now, just a post about a sign at Yellowstone that reads:

"Deep within the Earth, heated water dissolves and then transports silica, the same mineral found in sand and glass, to the surface.  During geyser eruptions, silica is deposited around narrow 'vents' or openings.  Over time this mineral, called geyserite or sinter, forms mounds of varying sizes and shapes."

The sign that's wrong about minerals.

EEeeeeeessssshh!!  If you zoom in on that photo above, you might be able to make out the text under the central picture of Castle geyser.  As to the science on the sign, the basic idea that hydrothermal fluids dissolve & reprecipitate silica is fine, and this sign probably communicates correct information to the reader for the most part.  However, it perpetuates a misconception in the understanding of what a mineral is.  "Silica", "geyserite", and "sinter" are NOT minerals.  At least, not in the geologic sense, and since this sign is communicated geoscience information, it ought to use geologic terms correctly.  

Silica is a chemical compound, with the formula SiO₂.  All minerals are chemical compounds, but chemical compounds are not necessarily minerals.  For one thing, minerals have to be solid.  So if silica is dissolved in water, it's not a solid, it's now a component of a liquid.  Using the term "mineral" in this fashion is a bit like the way the term is often used in nutrition, where various elements like calcium & iron are often referred to as "minerals".  They are sometimes referred to as "mineral nutrients" or "dietary minerals", but neither of these terms are very satisfying either.  I'm not sure why the term mineral ever got used in this fashion, since none of the "minerals" referred to in nutrition are minerals, they are simply elements.  But again, this sign is attempting to communicate geoscience, and in geoscience if something is dissolved in a liquid, it is most definitely not a mineral.  


Now suppose our silica is in a solid form, does that make it a mineral?  Not necessarily.  Several minerals are made of silica (quartz & its many polymorphs), but silica itself is not a mineral, it is a chemical compound.  The reason silica is not a mineral is because minerals are defined not only by their chemical composition but also by their atomic structure.  Quartz & all those other silica polymorphs each have a distinct atomic structure.  Silica can also form solid materials that are not minerals, such as opal.  Opal is a solid that does not have a crystalline atomic structure.  Glass is another solid material that also does not have a crystalline atomic structure.  A crystalline atomic structure means that the atoms are all lined up and bonded together in an orderly fashion that repeats itself in three dimensions thousands and millions and billions of times, depending on the size of the grain.  Non-crystalline solids are solids where the atoms are a bit more jumbled up & irregular.  So minerals are defined by their chemical composition AND their atomic structure.  Silica is a more general term that only means chemical composition, but doesn't specify the atomic structure.  


Geyserite is also not a mineral.  "Geyserite" is something of a generic term referring to the solid silica that is deposited around geysers.  So this is at least solid, but it still isn't a mineral.  Most of geyserite is the material known as opal, and as I already explained above, opal is not a mineral because it does not have a crystalline structure at the atomic level.  


Sinter is another term that really refers to the porous nature of the geyserite, so this is a term that's really about the physical attribute of the aggregation of the various grains of opal.  So really, this is a rock term.  


So what is a mineral?  That I'll save for another post.  


But why write this post?  Who cares?  I teach a course in minerals to undergraduate geology majors.  One of the most important concepts of the course is "what is a mineral?" and what is not.  Definitions, especially in science, are extremely important.  A geologist's understanding of the term "mineral" can't be gray & fuzzy; it needs to be precise & accurate.  Many geology majors grow up with an interest in natural phenomena & are likely to see signs like this one at Yellowstone, and they get these confused definitions in their heads.  In education, misconceptions (things we think we know but are actually wrong) are really, really hard to get out & get corrected.  


On the first day of my mineralogy class, I ask my students to simply list the name of every mineral they can think of.  When they took their introductory geology course, they learned about 20 or so minerals, so this exercise is intended to require them to recall that information.  But the answers given often include things that are not minerals.  Answers like "quartz, feldspar, granite, calcium" sometimes show up.  The first two are fine minerals, but #3 is a rock and #4 is an element and neither of them are minerals.  This shows that the students don't have a clear & precise grasp of what a mineral even is or is not.  In my experience, this is pretty typical for students at this stage of learning; hopefully at the end of the course they've got the concept mastered!  

But beyond the students in a mineralogy course, confusion about science abounds in our society.  A basic knowledge of the differences between minerals, elements, & chemical compounds is junior high level science.  So I cringe when these differences are misrepresented on a sign in a national park that's intended to communicate scientific information to the public.  The problem basically boils down to this: there's a precise, careful definition of the term that's used by those who know, and there's the loose, flimsy definition of the term that's used more in the general public.  A sign communicating geoscience to the public I think ought to be a bit better.