Welcome to rockPTX
Click here to read how P, T and X each played a role in forming the colorful rock pictured above
This particular sample is a metamorphic rock… an unusual ultra-high pressure (UHP) whiteschist assemblage from Mautia Hill, Tanzania, formed roughly 550 million years ago. The vivid blue mineral in the center is yoderite, a rare Mg-Al-silicate so far found in only two places in the world (both in Africa). Yoderite and its surrounding whiteschist (which is essentially a stable combination of talc + kyanite, two of the other minerals in the photo) only form at high pressures. In fact, at one time during its formation, this particular rock was at 700-800°C and 10-11 kbars (~35 km of burial depth).
That in itself may not seem so special; after all, everywhere in the world there are rocks much deeper than that beneath our feet, although we probably don’t often stop to ponder something so mundanely self-evident. What may be more interesting, however, is that before this rock become a whiteschist some 35 km down, it was likely just some surficial clay-rich sediment, perhaps dried and baking under the midday late-Precambrian sun. Such an inauspicious beginning for what is now a rare and colorful rock seems almost unfathomable. But in this case, tectonic forces were able to take that near surface mud & sand deep down, and the combination of heat & pressure fostered chemical reactions that eventually transformed the tiny clay particles into the coarse-grained and showy minerals now on display in the photo. Of course, this narrative is a fair bit oversimplified… as additional considerations, the original clays had to be of the appropriate mineralogical composition; the prevailing oxygen fugacity (a measure of how oxidized or reduced the rocks are) had to be high enough to stabilize yoderite; metasomatism may have played a role, at least according to some researchers; and also, undoubtedly, there were numerous intermediate reactions along the P-T path whose products simply weren’t preserved for us to discover. Nonetheless, even allowing for all of the real-world complexities, the sum transformation of clay to yoderite seems almost like a geological “Cinderella” tale.
While that transformation may be noteworthy enough, it’s still only half the story. For essentially all of Earth’s geologic history, subduction, over-thrusting and other mass movement of rock have buried once near-surface rocks. And most of these rocks probably never return to the surface for us to collect and study. However when, indeed even if, some of these buried rocks do return, many times the mineralogical evidence of the peak P-T conditions these rocks endured at depth has been largely or completely lost to re-equilibration, a process petrologists call retrograde reaction. But not so at Mautia Hill… at least, not entirely. Here, the combination of cooling & uplift that the whiteschist experienced to return to the surface occurred in such a fortuitous way (fast enough? dry enough?) as to prevent the peak high-P, high-T mineral assemblages from fully reverting back to some chemically-equivalent low P, low-T mineral assemblage (for example, the more common pair of clinochlore + quartz).
As is clear from the story so far, pressure & temperature play a central role in the formation of yoderite. But one of the questions unanswered so far is why do some rocks that follow a Mautia Hill-type deep & hot P-T path become whiteschists, while other rocks, perhaps even nearby or adjacent ones, do not? And an even more fundamental question, applicable to a much wider variety of metamorphic and igneous rocks, is just how the P-T path is even figured out in the first place? To address these questions, we have to look at our third parameter, that of composition (the “X”). Indeed, important aspects of composition relevant to this example have already been alluded to above: the mineralogy of the original sediments; the prevailing and perhaps varying redox conditions; and the potential role of mass transfer by fluids into and out of the system. So how can these seemingly disparate chemical clues help answer the question of how this rock may have formed? Well, the most important way is that these sorts of data provide valuable insight into deriving plausible mineral reactions. And at the same time, these sorts of data can also help dismiss reactions that are probably implausible (for example, those that could not occur at the presumed P-T conditions, or those that did not reflect the observed mineral assemblages).
Of the minerals discussed so far, from the starting clays to the yoderite, talc, kyanite and clinochlore, all the end-members fall within the MgO-Al2O3-SiO2-H2O chemical system (known by the acronym MASH). Those with an eye for chemistry might notice that because yoderite contains Mg (like talc) and Al (like kyanite), there’s likely a reaction that relates these minerals. And indeed there is! Add in the additional MASH components of quartz & water (and in this simplified example, ignoring the small amounts of Fe2+ and Fe3+ that naturally substitute for Mg and Al in yoderite), and this assemblage of minerals can be described by the reaction:
4 Mg3[Si4O10(OH)2] + 18 Al2O[SiO4] + 2 H2O ⇄ 6 Mg2Al6O2[SiO4]4(OH)2 + 10 SiO2
or, using mineral name shorthand:
4 talc + 18 kyanite + 2 H2O ⇄ 6 yoderite + 10 quartz
Out of the approximately 5000 known mineral species, there are only roughly a few dozen or so mineral end-members in the simplified MASH system, making the search for plausible whiteschist-relevant reactions a bit less daunting. Coupled with detailed field mapping and careful observations on thin sections & hand samples, the total number of reactions can be further whittled down. At this point, though, one might ask what is the value of being able to write out numerous possible mineral reactions? Well, with a thoughtful set of reactions, petrologists can use thermodynamic data to constrain everything from the activities of critical chemical components, to, you guessed it… where a reaction plots on a graph of temperature & pressure. And so here we’ve come full circle, back to how the 700-800°C and 10-11 kbars peak metamorphic conditions of this Tanzanian whiteschist were actually determined. (One important caveat that should be noted here is that for many less common minerals, complete thermodynamic data have not been measured yet. This deficiency sometimes turns out to be a major limitation to successfully making these kinds of calculations)!
Yet even more can be done. For example, this could include determining the overall P-T path of a rock. Here, instead of only ascertaining the single maximum P-T point, the path is the locus of all of the various P-T points the rock experienced throughout its full history. Deducing this path can be challenging, but may be accomplished in ideal cases by coupling peak P-T determination with both macro- and micro-structural data to establish the changing stress regimes during rock formation, examining trapped mineral & fluid inclusions to uncover earlier P-T points, and studying adjacent rocks of different mineralogies but of presumably similar burial histories to increase the number of (P-T)-dependent reactions that can be evaluated. Finally, with the addition of advanced techniques such as geochronology, thermochronology and geospeedometry to collect “time data” (assuming minerals appropriate for these measurements are present in one’s rocks), then even the age of the rock or the time frame of cooling, metamorphism or uplift can be determined. Indeed, a frequent expression encountered in papers on regional petrology is “P-T-X-t” or “P-T-t”, where the little “t” at the end is time. As the acronym seemingly suggests, that little “t” is typically the last piece of the petrogenetic puzzle, and it can sometimes be the most challenging parameter to figure out.
All things considered, this representative story is as elegant as the rock itself, a story where pressure, temperature and composition (each as they began, and also each as they changed over time) all played an important role. Exploring the dynamic natural processes within the Earth that can bury plain old mud and then return it to the surface to end up one day as an outcrop of rare whiteschist is only one of countless areas of research that makes geology such a fascinating science! And while some rocks such as this yoderite-bearing whiteschist might be a bit more exotic than the average granite or gneiss underfoot, every igneous, metamorphic and metasomatic rock (and yes, even every woefully uninspiring sedimentary rock… lol) has some unique story of P, T and X to tell.
So please feel free to explore the pages here. Although the scientific content is probably of greatest interest, you might like to check out the accompanying about me, travel and recipes pages (all under the time out tab), to discover both a bit about my background and professional interests, and a bit about a couple of diversions I enjoy when I’m not thinking about rocks. Getting into the geology, the thin section scans page introduces the most extensive content so far, with over 250 image pairs (in unpolarized light and between sheets of crossed polarizing film) of high-resolution entire thin section scans of rare & exotic rocks; so far, the figure captions are variably complete, but they are being updated regularly. The optical microscopy and electron microscopy pages both have limited content so far, but they are also being actively expanded. Lastly, the optical mineralogy tutorials, “how to” PDFs and digital books pages (all under the continuing education tab) offer advanced original content covering subjects such as the optical properties of rare minerals, balancing complex mineral reactions, microprobe analytical strategies, and normalizing microprobe analytical data. This additional content is geared specifically towards graduate and advanced-level undergraduate students with research interests in mineralogy or petrology, and is intended to enhance their understanding and appreciation of the science.
Learn… and enjoy! 🙂
Although the site is still very much “under construction”, feel free to poke around… at this point, most of the pages have some content, and I’m routinely adding new content, and exploring ideas for new pages & general site improvements.