"Rock of the Month #165, posted for March 2015" ---

A "gypsum rose"

from northern Laguna Madre, near Aransas Pass, southeast Texas

In terms of mineral form ("crystal habit"), a number of minerals are apt to be described as occurring in rose or rosette form. Other examples include: sulphates (barite); carbonates (azurite); arsenides (safflorite); graphite and molybdenite; and ice (snowflakes). In the case of gypsum, it seems that as the sulphate cements itself around the sand, it retains crystallographic integrity, while the sand serves as a reinforcement. In another sample, it appears that tabular crystals lie side by side, imbricated (like tiles on a roof). Sandstone cemented by gypsum generally forms under arid or semi-arid conditions. The gypsum crystal masses may enclose as much as 60% sand (Greensmith et al., 1971, p.132).

Gypsum is hydrated calcium sulphate (CaSO₄.2H₂O). It has a very variable habit, including crystals, fibrous and massive forms (Grice, 2010, pp.124-127). It is very soft (Mohs hardness of 2, softer than a fingernail), and light (specific gravity 2.3). The fibrous form is known as selenite or satin spar, while alabaster is the massive form.

Evaporation of saline solutions in desert sand can result in the growth of mineral crystals in a shallow sub-surface layer that may exceed 3 metres in thickness (Stoppato and Bini, 2003, pp.35,39,69). In the case of gypsum, complex crystal forms may result, generally in playa and sabkha environments. Examples are known in Tunisia, Algeria and the Dead Sea. Gypsum aggregates formed in this manner may be termed desert roses. They occur also in Peru (Moore, 2012), Alberta (Mussieux and Nelson, 1998), the Red River floodway of Manitoba (Grice, 1989) and elsewhere.

Gypsum is a mineral often found in evaporite facies sediments, that is, in hot coastal settings with restricted water flow. Elements of this sedimentary environment can be compared with carbonate-hosted lead-zinc mineral deposits of the Mississippi Valley type (MVT deposits: Beales, 1977). Solution breccias and gypsum layers have been documented in deposits such as Pine Point in the Northwest Territories (ibid., pp.58-59). Such MVT deposits in the Paleozoic strata of the Rocky Mountains have sedimentary features that can be compared to modern sediments in coastal settings such as the Laguna Madre and the Persian Gulf (Manns, 1982). Some disconformities in the stratigraphic record may be explained in terms of the prior existence of soluble or reactive cements such as the gypsum in the specimen from Laguna Madre. Sulphates such as gypsum and especially barite are highly insoluble in water, so a pure groundwater would not be an effective solvent.

References

Beales,FW (1977) Stratigraphical approach to Mississippi Valley-type ore deposits. In "Ore Deposits Workshop", Geology Department, University of Toronto, I-39 to I-90.

Greensmith,JT, Hatch,FH and Rastall,RH (1971) Petrology of the Sedimentary Rocks. Thomas Murby & Co., London, Revised 5th edition, 502pp.

Grice,JD (1989) Famous Mineral Localities of Canada. National Museum of Natural Sciences, Ottawa, 190pp.

Manns,FT (1982) Stratigraphic Aspects of the Silurian-Devonian Sequence Hosting Zinc and Lead Mineralization near Robb Lake, Northeastern British Columbia. PhD Thesis, University of Toronto, 252pp.

Moore,T (2012) Ste.-Marie-aux-Mines show 2012. Mineral.Record 43, 616-624.

Mussieux,R and Nelson,M (1998) A Traveller's Guide to Geological Wonders in Alberta. Federation of Alberta Naturalists and Can.Soc.Petrol.Geol., Calgary, 254pp.

Stoppato,MC and Bini,A (2003) Deserts. Firefly Books Ltd, Toronto, 256pp.