Salt of the Earth

On the 8^th of August Darwin took a geological excursion upstream of Patagones to a local salt deposit he called a “Salina”.  This site consisted of a large briny lake that evaporated in the summer months, leaving behind salt deposits which were then mined by the local residents.  Darwin describes the lake:

“Rode to the great Salina, which is worked for the exportation of its salt, it is situated about 15 miles up & 3 from the river, —at this time it is nothing more than a large shallow lake of brine; but in summer it dries up & there is left a large field of snow white salt. — Both on the banks of the river & on the edge of the lake there were heaps of many hundred tuns ready for exportation. — The working time is as it were the harvest for Patagones, the whole population encamps on the bank of the river & every morning with the bullock waggons the men go to the lake to draw out the salt & form the Montes. There are other Salinas which are more distant & these are many leagues in circumference & the salt several feet thick, a quantity sufficient to supply the world.” (Aug 8)

The “great Salina” is located exactly where Darwin describes it in his diary. Zoom in on the large whitish patch located northeast of Viedma (about 15 miles upriver) on the map below.  You’ll see a big salt flat.  If you zoom out and look about 100 miles to the west of Viedma you will see the much larger and “more distant” salt deposit Darwin also mentions – the Salinas del Gualicho.

The Salinas del Gualicho (posted by Gregorio Raul Unfire):

Although Darwin suggests that there is enough salt to “supply the world”, he also goes on to note that these salt deposits are not ideal for all purposes, and hence, are only used locally  He talks about this briefly in his diary, but provides a more detailed assessment in Voyage of the Beagle:

“This salt is crystallized in great cubes, and is remarkably pure: Mr. Trenham Reeks has kindly analyzed some for me, and he finds in it only 0·26 of gypsum and 0·22 of earthy matter. It is a singular fact, that it does not serve so well for preserving meat as sea-salt from the Cape de Verd islands; and a merchant at Buenos Ayres told me that he considered it as fifty per cent. less valuable. Hence the Cape de Verd salt is constantly imported, and is mixed with that from these salinas. The purity of the Patagonian salt, or absence from it of those other saline bodies found in all sea-water, is the only assignable cause for this inferiority: a conclusion which no one, I think, would have suspected, but which is supported by the fact lately ascertained, that those salts answer best for preserving cheese which contain most of the deliquescent chlorides.

The border of the lake is formed of mud: and in this numerous large crystals of gypsum, some of which are three inches long, lie embedded; whilst on the surface others of sulphate of soda lie scattered about. The Gauchos call the former the “Padre del sal,” and the latter the “Madre;” they state that these progenitive salts always occur on the borders of the salinas, when the water begins to evaporate.” (Voyage of the Beagle)

So what is salt?  It may not be as simple as you might think.  We tend to think of “salt” as the table salt that we put on our food.  Table salt is actually the mineral halite which has the chemical formula NaCl (chemists would call it sodium chloride). But in chemistry “salts” are any ionic compounds that are created by the reaction of an acid with a base. Sea water is made up of about 3.5% dissolved salts, while “brine” in considered to be water with more than about 5% dissolved salts. Even fresh water has some dissolved salts in it – it is this water that ultimately carries salt to the ocean where it becomes more concentrated.

Diagram showing the dissolved salts in sea water (from Wikipedia Commons):

When seawater or a salty lake evaporates, the ions dissolved in the water can become more concentrated and start to crystallize and settle out of the water.  (In sea water this typically starts for some minerals when about 50% of the water has evaporated.  Halite does not crystalize out until about 90% of the water is gone and the sdium and chlorine are really concentrated.) When these crystal sediments settle to the bottom of the lake (or remain after the water has dried up) they are called evaporates – a generic name for a group of sedimentary rocks that form in this manner.

There are several different types of evaporite rocks.  For example, as seawater evaporates one of the first things to crystallize out is the mineral calcite (calcium carbonate) – which accumulates to form the rock limestone. Then, as the concentration of salts becomes higher the mineral gypsum (calcium sulfate) crystallizes out forming rock gypsum. (Recall that Darwin’s analysis of the evaporites at this location also found gypsum.) Next to crystalize of the water is the mineral halite (sodium chloride) which forms rock salt.  And lastly, a number of minor salts, such as potassium and magnesium salts will settle out the evaporating water (these include things such as such as saltpeter used in gunpowder, or sylvite – a form of potassium chloride). Other evaporite minerals that form in briny lakes (and which you may have heard of) include trona (a more complex sodium carbonate mineral) and borax (a sodium borate).

Some evaporate minerals (compiled from Wikipedia Commons), including (a) calcite, (b) gypsum, (c) halite, (d) sylvite, (e) trona, and (f) borax. Note that color in these minerals can vary depending on impurities that get mixed in with the crystals. ALl of them can appear white.

Salt, in its many forms, has played an important role in human history – primarily as a dietary supplement and in the preservation of food.  It has had many other uses (such as salting roads to help melt ice or use in industrial processes). One of my favorite historical uses was the use of salt as a form of payment for Roman soldiers (their salarium) – a practice that led to the term “salary”. (There is some debate on the actual derivation here.  Some suggest in may have been derived from the currency paid to the soldiers in order to purchase salt. But you get the picture…)

In the end, it was overall a pleasant day for Darwin and he wraps up his diary with:

“Many of the geological facts connected with this Salina are curious & I returned highly satisfied with my ride.” (Aug 8)

More on some of these other geologic observations, as Darwin’s speculations on life within the salt,  in the coming days… (RJV)