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.

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