Gypsum
occurs generally in the form of cavity fillings. Amorphous forms are in massive
cluster and scales. Zooming this part, rapidly crystallized interpenetrating
twinned platelets and needles of sulphate crystals are seen. The sizes of
gypsum crystals vary from 10×2 to 20×3 µm. The length and breadth ratios vary
around 6. A lot of empty spaces and voids are seen around the crystals. Most of
the crystals are euhedral in form. The smaller the gypsum particles, the higher
the peak of supersaturation reached before precipitation occur. The use of
smaller particles of calcite in the feeding suspension also leads to a solid
product in which the particles precipitate from the solution. In the sequence
of crystallization from the evaporation of seawater, it is known that
carbonates deposits first at is 50% of volume reduction followed by metastable
solid phases of CaSO2. 2H2O at 20% of volume reduction by
evaporation 25. In this case, the change from metastable to stable
paragenesis occurs in early diagenetic phase. Sometimes pseudomorphs after
gypsum occur. When the volume of seawater shrinks to 10% of original volume,
halite begins to crystallize. Scales of gypsum precipitated at super-saturation
by mixing of groundwater and estuarine water and also by changes in
temperature, pressure, salinity levels, pH and CO2 29. A linear
variation of chemical constituents of these minerals indicates that these
minerals are crystallized from common source of pore fluids but with repeated
dehydration and hydration processes which widened the hairline cracks
significantly in the course of time duration. Further stability conditions of
these minerals indicate that increasing degree of hydration is related to
increasing pore-fluid pressure due to expansion of porevolume with decreasing
temperature in an environment of closed system of evolution within pore spaces
by repeated influx of pore fluids and their subsequent evaporation in the
course of time.