Playing hide-and-seek in a gypsum quarry

Believe it or not, but rock quarries can be much more exciting than the most popular tourism objects and a lot more impressive than the most beautiful cliffs.

Salaspils gypsum quarry (photo: L.Spruzeniece)

In Latvia I live in a place not so far from an active gypsum quarry. So, last summer finally the day came when I decided to take a closer look on it. After a short examination of GoogleEarth, the best access route was found. Then, some 40 minutes on a bike and I was there. I did not see any prohibition signs at the entrance, but it also did not feel right to show off too much as there were excavators working inside. So it was kind a tricky to do much research. I did not succeed to access the exposure of the quarry wall as I had a hard time with hiding from excavators but anyway, there was a lot to see even along the quarry edge.

My “sandbox” (photo: L. Spruzeniece)

In a nutshell gypsum is an evaporite mineral. It forms in arid and semiarid environments by the evaporation of shallow marine basins. The most common evaporites are gypsum (CaSO4·2H2O), anhydrite(CaSO4) and rock salt or halite (NaCl). All of these minerals can form in the same basin by gradual increase of the water salinity. Experiments by Usiglio (1849)1 have shown that if you take a basin with a normal sea water and evaporate 50% of it, the carbonate minerals starts to precipitate (limestone and dolomite), when only 20% of the water is left, you get gypsum and anhydrite but at the 10% of original water volume, halite appears. By continued evaporation some even more rare magnesium and potassium minerals precipitate.

What did I see?

I found 3 types of gypsum at the quarry:

1) fibrous gypsum or satin spar

2) flaky gypsum

3) slaty gypsum

Fibrous gypsum. The purest of all the gypsum types here (about 99% gypsum). Characterized by a fibrous structure, silky or satin-like luster, hardness of 2 on the Mohs scale (can be scratched with a fingernail). There is a celestine cone (the gray one) in this specimen (photo: L. Spruzeniece)

Often the fibrous gypsum had a color of salmon. In this specimen it is a bit deformed – the fibers are bended and the gray line in the middle (suture) is broken on the right side (photo: L. Spruzeniece)

Flaky gypsum (the brown one) alternating with fibrous gypsum (the white one). It is easy to see the thin flakes in a characteristic rosette pattern at the layers of flaky gypsum (photo: L. Spruzeniece)

The imprints of gypsum rosettes preserved on the bed surface (photo: L. Spruzeniece)

Slaty gypsum with various deformation patterns, probably made in dehydration processes at the time of sediment formation (photo: L. Spruzeniece)

Slaty gypsum. The lower part consists of thin clayey (and probably carbonatic – I did not have an acid with me to check) layers (photo: L. Spruzeniece)

What does it mean?

The gypsum at this quarry formed in the late Devonian, the time when Latvia was located near the equator and covered by a shallow sea. The undulations on the seafloor prohibited water exchange with the ocean so the sea water had a high salinity level. The intensive evaporation took place here because the climate was hot. That led to the precipitation of gypsum. Then there were also dolomite layers at some places. That means the periodical additional water inflow in the sea and the decrease of salinity. Also the clay layers testify of additional influx of material. Probably the clay particles came from the newly formed Caledonian mountains in Scandinavia (just guessing). And then the deformation patterns – seems like some of them (especially the ductile ones in slaty gypsum) was made by the dehydration processes at the time of deposition in Devonian while other – the brittle ones in fibrous gypsum could have resulted by the pressure of advancing ice sheet, not so long ago, at the last Ice Age.

References:

1Maurice E. Tucker (2001) Sedimentary petrology: an introduction to the origin of sedimentary rocks.

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