Sand Roses of Saudi Arabia,

by Denis Mougenot

This page is a mirror of a paper published in the The Oil Drop
the monthly newsletter and an electronic publication of the 
Dhahran Geoscience Society.

 Dhahran Geoscience Society

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Gypsum is a common mineral (CaSO4 + 2 H2O) that is dissolved in the sea water, and crystallizes by the concentration calcium sulfate through evaporation; therefore, gypsum is one of those evaporite lithologies that are frequently found in the geological record. In Saudi Arabia, gypsum outcrops in the Miocene formations along the coast of the Red Sea where it is extracted for cement (Yanbu). In the Gulf area, gypsum deposits occur in sabkhas as fine, white interbedded layers. At some rare locations, it crystallizes in the form of sand roses Figure 1, one of the many gypsum crystal habits that explain its popularity among mineral collectors.
 

In the Eastern Province of Saudi Arabia, sand roses can be found in a variety of shapes, sizes and colors that depends on the characteristics of the sand in which they grow. Since sand roses are found at the surface, most of the people think they originate there. In fact, sand roses are not pebbles shaped by wind, but crystallize at the water table, about 1 m below the surface, where large, fresh crystals can be found. Different mechanisms could explain the outcropping of sand roses at the surface, but their appearance to daylight is short lived. After weathering, erosion and dissolution, calcium sulfate re-enters the aquifer where it will contribute to form new delicate and ephemeral sand roses.
 

 

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Some unique sand roses: No one sand rose is identical to any other. They are assemblages of bladed gypsum crystals that thin towards the edge, where the blade becomes transparent and sharp. In certain circumstances, prismatic crystals with plane facets replace these blades (black gypsum of Half Moon Bay). Since gypsum crystallizes in the pore spaces of coarse sand, its growth is driven by the complex organization of the porosity network. The main trend is the horizontal stratification of the sand that also conforms to the surface of the water table. This preferential plane explains the growth of sand roses from well-developed horizontal blades, together with the clustering of rosettes in large horizontal plates Figure 2. Some rosettes and clusters display blades that seem to have developed in various directions along crystallographic axis; however, the rotation of the rosettes in the loose sand during their growth is also to be considered. In some particular circumstances sand roses look remarkably like a spherical rose flower with its petals wide open. Since pure gypsum is transparent, sand roses are the color of the sand in which they are embedded, usually dark brown. If the sand is coarser, sand rose substance looks like honey. If the sand is black, due to high content in organic matter, the sand rose is black too. When a sand rose outcrops and starts weathering, the embedded sand grains become lighter as they oxidize, and the gypsum matrix turns white and soft. After a few years, depending on its size, the sand rose becomes powder and returns to the ground.
 

Sand roses are found in a variety of sizes, ranging from small rosettes a centimeter across to large clusters, more than one meter in diameter. Some samples are built from a few very large blades while others are made into an artistic assemblage of spherical rosettes. All of them provide various natural sculptures that most of us might be pleased to display at home. Such large samples are not unusual for gypsum and explain how attractive the search for sand roses can be. The most fascinating aspect is the possibility of being able to find all these different shapes and sizes from the same small garden. Each visit to the sabkha holds a surprise. Nevertheless, the unexpected large and fresh sand rose clusters rarely lie at the surface. Hard digging is necessary to reach the water table and your pain is not over. The best samples are still deeper and could be very difficult to extract because of the suction of the wet sand on the contorted surface of the rose. These natural treasures are worth your efforts.
 

 

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Ephemeral sand roses: The sea is the source of brine, rich in calcium sulfate that seeps underground into sabkhas. Coarse sand is the substance of sand roses, but you will never find sand roses below the surface at the beach, because the concentration of brine is not high enough at the shoreline (however in some circumstances, gypsum may precipitate in open water conditions). On the supratidal zone or in areas of limited marine influence, where the salinity of the brine increases to more than three times that of normal sea water, gypsum formation can start. When the water table is about 1 m below the surface of a sandy sabkha, ideal conditions for controlled evaporation occur that permits sand roses to develop Figure 3. If the water table is deeper, the evaporation is not strong enough for gypsum to precipitate. If shallower, salt will crystallize and prevent the sand rose from growing. Further landward, sand roses can be found also in the Rub-Al-Khali as well as in most of the sand desert areas, where ancient brine, not only sea water, is the source of gypsum.
 

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 from Chris Heine
 

 

The coast of the Eastern Province of Saudi Arabia, south of Dhahran, offers ideal conditions for sand roses to crystallize. A north-south elongated belt of eolian sand reaches the coast between Half Moon Bay and Al-Uqayr. The brine of the Arabian Gulf seeps into this eolian sand, pumped by the evaporation at the surface of the sabkhas Figure 4. When it isolates from the sea, the brine concentrates. As the water table rises and falls with seasons, the gypsum crystal grows in the pore space, trapping sand grains. In the early 90's, analyses performed by KFUPM confirm this mix between quartz (about 10%) and gypsum (about 65%), with some other evaporites (e.g. salt and dolomite). This proportion of gypsum is more than the porosity of loose sand and demonstrates that gypsum is not only growing in the pore spaces. It is also shouldering aside the sand. The lower concentration in silica at the periphery of sand roses, evidenced by KFUPM, supports such mechanism.
 

How can these crystals originated at the water table be found at the surface of the sabkhas? Between the Dhahran airport and Half Moon Bay, a salt flat has its surface paved by thousands of isolated prismatic gypsum crystals. Most of them are white and split along the cleavage planes Figure 5. Some are still entire brown crystals and a few are well-preserved black gypsum crystals, the color of the wet, organic dark sand in which they originated at least 50 cm below the surface. Such an accumulation is unusual and the enthusiastic geologist will be disappointed to see crystals becoming scarcer when digging down to the water table. 

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Obviously there is a mechanism transferring gypsum crystals to the surface from where they originate. The most important process is probably lowering of the surface by wind erosion. Another is the seasonal fluctuation of the water table, which explains the occurrence of the dark sand and black crystals up to 50 cm above the water table. Finally, it is meaningful to speculate about the existence of convective motions that are common in the loose sediments of the ground. All these mechanisms may contribute to the outcropping of prismatic gypsum and sand roses Figure 6
 

Sand roses probably form rapidly (in tens of years) and they do not last long. They are short-lived creations in geological terms, since there are no such things as fossilized sand roses. If not dissolved, sand roses get so transformed that they are difficult to recognize (e.g. in the Permian nodular anhydrites of Texas). From this point of view, sand roses are very similar to the polymetallic nodules found on the deep ocean floor. These nodules or cluster of nodules, similar in size to gypsum rosettes, are concentric layers of iron and manganese hydroxides that precipitate around seeds (could be a shark's tooth or the cap from a Coca-Cola bottle). Unfortunately for miners, polymetallic nodules are dissolved when buried under soft sediments, which results in their absence from the geological record. Overall, these two quirks of nature, one continental - deposited at the top of the water table -, the other marine - deposited at the base of the water column -, are ephemeral witnesses of the intense geological processes that occur at the interface between water and sediments.