Tuesday, February 27, 2018

The (XRD) Results Are In!


Huzzah! The XRD results are in!

If it weren't for my e-mail exchange with Mark at the beginning of the month, some of the mineral assemblages would have certainly confused me.

Recall: I divided my samples into four categories
  • Solid - in tact, relatively hard crystalline diapir material
  • Intermediate - softer diapir material, can be crumbly
  • Crusty - very friable, vuggy material found on diapir surfaces, often adjacent to intermediate rock
  • Surficial - salts precipitating on soils and rocks downstream of salt diapirs
My hypothesis was that the "solid" diapir samples would be composed of anhydrite, the "crusty" samples would be gypsum (anhydrite that has been aqueously altered), that the "intermediate" would be... maybe both? Given the strong gypsum/anhydrite signatures of secondary salts in the ASTER TIR data, I hypothesized that the surficial salt would be gypsum with halite (after identifying some secondary halite in the field).

Let's start simple.

Here are what some of the XRD analyses show:
  • Solid samples - Anhydrite + Gypsum (more anhydrite than gypsum)
  • Intermediate - Gypsum
  • Crusty - Gypsum, sometimes with traces of quartz or calcite
At first glance, these results are fairly close to the hypothesis, although I had thought the solid samples would be anhydrite only.

A few of the solid samples, though, were not anhydrite. One was calcite (i.e. limestone) with secondary gypsum, and another was dolomite with secondary gypsum, trace quartz and other minerals. This is not alarming. The literature describes the diapirs containing "subordinate limestone, [and] rare dolostone," (Harrison and Jackson 2014). In hindsight, these two rock samples look more like a carbonate than anhydrite or gypsum, but no better "solid" rock exposures were present at these outcrops (Whitsunday Bay Diapir, Strand Diapir).

What surprised me more, though, are the compositions of the surficial salts. 

These samples proved very diverse. Often, they include traces of non-salt minerals (i.e. quartz, clays) but these are extremely likely to be contamination from the soil. It was difficult to scoop up bits of precipitated salt without getting a little dirt or sand mixed in too. So it is not surprising to see common sediments. However, the compositions of the salts were very varied, with:
  • Pure halite
  • Gypsum with mirabilite and dolomite
  • Gypsum with thenardite
To be honest, if it weren't for the e-mail exchanges with Mark, I would not have even thought of checking for thenardite or mirabilite. These minerals are Na2SO4 and Na2SO4·10H2O respectively. Nesse (2012) explains that thenardite can be found in saline lake evaporite deposits, and can be found as an efflorescence on soil. I had to look up what efflorescence means, but it effectively describes the means by which our secondary surficial salts are precipitating on soils and rocks on Axel Heiberg Island. It makes sense that mirability, the hydrated form of thenardite, would be found in the similar settings. Now, unlike halite, thenardite does not have an isometric crystal structure - it is orthorhombic. The implications of this are that thenardite would have spectral absorption bands. I think I should look them up, and see if they are similar to gypsum or not - will thenardite salts be disguised as gypsum in our spectral images? Or can we use different spectral bands to isolate thenardite secondary salts from gypsum secondary salts? Future work will tell, but for now I'm just going to work on finishing my thesis.

P.S. One final highlight -



I have been told that this is exceedingly uncommon. Nesse (2012) says that some samples of halite may be fluorescent (anyone have a UV light?) but that does not necessarily explain how the white powder would turn into a dark grey powder permanently after being run through the XRD machine. Maybe the halite contains radiation-sensitive impurities? Maybe I shouldn't have been licking it in the field?  Who knows. I'm definitely curious, and the XRD technician is also interested in investigating this phenomenon. 



Harrison, J.C., and Jackson, M.P.A. 2014. Exposed evaporite diapirs and minibasins above a canopy                  in central Sverdrup Basin, Axel Heiberg Island, Arctic Canada. Basin Research, 26: 567–                    596. doi:10.1111/bre.12037.

Nesse, W.D. 2012. Introduction to Mineralogy: Second Edition. Oxford University Press, New York.

Tuesday, February 6, 2018

Some salty waters: Thenardite, halite, and gypsum


In my drafts and writing revisions, I noticed that I had a few missing pieces of information.

To clarify a few questions I had regarding the chemistry of perennial springs on Axel Heiberg Island, I reached out to Mark Fox-Powell, who joined us in the field last year. Mark is a post-doctoral research fellow at the University of St. Andrews, with a background in microbiology. His current research is in astrobiology, with emphasis on geochemistry of natural waters.

On our trip, Mark sampled the water and precipitates in and around perennial springs. He is using these samples to analyse their water chemistry, and to produce visible and shortwave infrared spectra of the precipitates. Existing spectral databases of hydrated sulphate and chloride salts are derived from pure minerals produced in controlled laboratory conditions. By using the salts from perennial springs, his team will be able to measure the spectral signatures of impure, naturally occurring salts.

The ultimate goal is to use terrestrial salts as an analogue for "non-icy" materials on Jupiter's moon, Europa. Europa is an ocean world. Its surface is a shell of ice of unknown thickness, over an ocean thought to have the potential to support life. For this reason, Europa is the target of the next NASA flagship mission Europa Clipper which will launch in the early to mid 2020s, and will carry instruments to image its surface at higher resolution. While still unknown, the non-icy materials identified on Europa are hypothesized to be salt precipitates - by understanding the spectral properties of naturally occurring terrestrial salts under Arctic and European conditions, we may be able to better constrain which salts are occurring on Europa. By gaining insight into what chemical materials are present in Europa's waters, astrobiologists will have a better understanding of what kind of life could potentially inhabit these oceans.

So, what did Mark and the team at St. Andrew's find?

We visited three perennial springs during our 2017 Axel Heiberg Island field season. These were Lost Hammer Spring (north of Wolf Diapir), Stolz Springs (emerging from Stolz Diapir), and Colour Peak Springs (southern base of Colour Peak).

Aerial view of Lost Hammer Spring, north and downstream of Wolf Diapir. 

The main vent of Lost Hammer Spring is a >1 m high accumulation of mirabilite and thernardite. Halite precipitates in the surrounding white areas. There is evidence of seasonal layering within the vent.
Lost Hammer Spring has been previously studied by Western alumni Melissa Battler (2013). The water chemistry analysis falls in line with these data, with the spring water being dominated by sodium and chloride. Interestingly, it has the highest sulphate concentration of the springs we visited. Some of these sulphates are precipitating as mirabilite (Na2SO. 4· 10H2O) or thernardite (Na2SO4)  rather than gypsum (CaSO. 4· 2H2O) or anhydrite (CaSO. 4), though. I'm planning on looking into sodium sulphates to see if they have similar or different spectral signatures than their calcium sulphate counterparts.

Segment of the very extensive perennial springs emerging from Stolz Diapir. According to Mark's analysis, the white minerals are dominated by halite and hydrohalite, whereas the darker, greyish minerals are predominantly mirabilite and thernardite
Stolz Springs has the highest concentration of chloride in its waters. This makes sense, given that Stolz Diapir has an outcrop exposure of halite at surface. Different parts of the spring deposits are dominated by halite, and others mirability/thernardite.

Perennial spring at the base of Colour Peak. The dark terraces are calcite+gypsum spring precipitates. The white minerals are halite forming at the edges of the springs.

Colour Peak has multiple spring outlets, which have appear to have lower chloride concentrations than the other sites. The dark terraces are only present at Colour Springs, and are made up of a combination of calcite and gypsum. There are also halite crystals precipitating on the soils adjacent to the terraces. If the terraces contain gypsum, then they certainly are contributing to the strong gypsum signature in our ASTER TIR images downslope of Colour Peak. The streams are very smooth compared to Colour Peak itself, which fits our hypothesis and radar observations!

One of the main takeaways here is that there are certainly more sodium-sulphates around Lost Hammer and Stolz springs than I thought.

Digging through some literature, Howari (2004) writes that thenardite has absorption features at 1.5, 2.0, and 2.3 µm due to the inclusion of water molecules. The latter two are very similar to the absorption features in gypsum at 1.9 and 2.2 µm. Similarly, although crystalline halite does not produce any notable spectral signatures, when aqueous it can also absorb at 2.0 µm from trapped water. Similarly, Howari et al. (2002) write that the SWIR signatures of thenardite can obscure that of gypsum when both are present in soils. This implies that thenardite, halite, and gypsum might look similar in our visible-near infrared and short-wave infrared and composite images. 

Stuff to consider.

I'm going to get back to writing.

Battler, M.M., Osinski, G.R., and Banerjee, N.R. 2013. Mineralogy of saline perennial cold springs on Axel Heiberg Island, Nunavut, Canada and implications for spring deposits on Mars. Icarus, 224: 364–381. doi:10.1016/j.icarus.2012.08.031.

Fox-Powell, M.G., Osinski, G.R., Gunn, M., Applin, D., Cloutis, E., and Cousins, C.R. 2018. Low-Temperature Hydrated Salts on Axel Heiberg Island, Arctic Canada, as an Analogue for Europa. In 49th Lunar and Planetary Science Conference. Lunar and Planetary Institute, Houston. p. Abstract #2564. Available from http://www.lpi.usra.edu/meetings/lpsc2018/pdf/2564.pdf.

Howari, F.M., Goodell, P.C., and Miyamoto, S. 2002. Spectral properties of salt crusts formed on saline soils. Journal of Environmental Quality, 31: 1453–1461. American Society of Agronomy, Crop Science Society of America, Soil Science Society.

Howari, F.M. 2004. Chemical and Environmental Implications of Visible and Near-Infrared Spectral Features of Salt Crusts Formed from Different Brines. Annali di chimica, 94: 315–323. Wiley Online Library.