Surface-Subface Geochemical and Mineralogical Study of Gypcrete in Alexandria Area Central Iraq

Gypsiferous soil deposits (Gypcrete) are weakly consolidate earthy mixture of secondary gypsum, sand and clay. It is formed in arid and semiarid area with annual precipitation rainfall less than 400mm. These sediments occur in surface and subsurface in region of little rainfall and rapid evaporation. This research deals with the study of gypcrete in Alexandria to improve the mineralogical and geochemical properties of the gypcrete. The gypcrete soil is used as raw material to produce the plaster for building purposes. Three samples of gypcrete were chemically and geochemically analyzed. The common mineral is howed in 0-0.5m Gypsum followed by Calcite in 0-1m and Quartz in 1-1.5m due to leaching and infiltration by rainfall as well as it full the clay Matrix and the voids between soil constituent, therefore soil properties changes with depth and the clay materials increases such as (Al, Na and K oxides) and Gypsum compound decreases (Ca, S and Sr oxides), yet the change in element concentration, takes the same path. This indicate that annual season rainfall is the only factor affecting mineralogy in this arid area and this means that the Gypsum concentration cannot be affected by ground water either increasing or decreasing. This research concerns about mineralogy and element occurrence variation in this material with accordance to their concentration from surface to depth of 1.5 meter.


Introduction
Gypsum is one of the important non-metallic minerals, composed of hydrated calcium sulphate (CaSO 4 .2H 2 O).It contains 79.1 % calcium sulphate, 20.9% water [1].Primary gypsum within Al-Fatha Formation has been precipitated by sea water, while secondary Gypsum formed by dissolving primary gypsum rocks at source area and redeposition at younger formation as sediments in Holocene.[2].The gypsif-erous soil (Gypcrete) usually developed in arid and semi-arid area with less than 400mm annual rainfall where abundant sources of gypsum exist and gypsum accumulation Open Access in the soil are related to quaternary deposits more than 1m and in the soil if the depth of the water table is less than 5m [3].The gypsiferous soil in Iraq isrelated mainly to the substratum containing gypsum and had prepared soil distribution for the first time, secondary gypsum is concentrated in the middle third and south of Iraq [4].The main reason for gypsum accumulation in the soil is precipitation from ground water and surface water by strong evaporation.A large part of gypsum accumulation occurs as sub soils, finegrained soils contain more gypsum than coarse grained soils.Almost all gypsum accumulate above capillary water zone in dry areas at which water table is located (2.5-3) m from surface, ground water will evaporate [3,4].The gypsiferous soil classif-yed into two types, the first type is the soil which has been subjected to leaching process, removing a large part of gypsum and soluble salts leading to change the distribution of them.The second type represented soil that not subjected to complete or partly leaching process and their distirbution remained unaffected [5].Petrographic study and XR.D study showed that gypcrete consists of gypsum, quartz, feldspar, Calcite, biotite, rock fragment and heavy minerals.Chemical analyses showed that the content of gypsum in the gypcrete in Alexandria locality in Babylon governorate is not suitable for production of a good quality plaster.Table3.Three samples were selected from seven sites at the depth (0-0.5),(0.5-1), (1-1.5)mshowed that gypsum was dominated mineral in the studied area followed by calcite in the (1-1.5)mbut the percentage quartz increases with depth to 13.2% in the depth (1-1.5)m [6,7].The oxides of high percentage in the surface (0-0.5) in (Ca,Sr, S oxides) decreases with depth, but (Al, Na, F ,K, Mg, Ti, C oxides) increases because these are mainly cons-tituents of soil which increases with depth compared to gypsum minerals [8].Element analysis chemically showed that concentration varied with depth (As, Bi, Hg) decreases while (En, Er, Dy, Cs, Cr, Ce, Ba, C, Nd, Nb, Lu, La, Ho, HF, Cd, Ca, Th, Tb, Ta, StSn, Sm, Pt, Sb, Zr, Yb, Y, W, V, U, Tm, TI(etc) increased [9].Table 4.This can show the origin of secondary gypsum in the studied area formed by dissolving primary gypsum and redeposition from the surface water at younger formations in the arid and semiarid area that means effect of ground water is minor or neglected.[10,11].The aim of the present research is to study the relation between distribution of elements with depth and their relation to environment, also to confirm the formation of gypsum in this area.

Materials and Methods:
The field work of gypcrete soil was carried out in Alexandria including sampling of the gypcrete soil from seven sites by selection of 3 samples collecting according to change in gypsum content or lithological chang-es.(A1=(0-0.5m,A2= (0.5-1m, A3=1-1.5m)crushing the samples with notification of inhomogeneous crushing then keeping them it in plastic bags labeled.Theses samples are suitable for chemical and mineralogical analysis after figuring .

Petrographic study
Because of the variable nature of most gypcrete sample impregnated in (araidite) (AY105) and hardness (H Y 930) following the method of moorland (1968 in Hatchison 1974) and samples were collected by using point counter mechanical stage as suggested by chayes (1949).This work was done In the Laboratory of Collage of Science Department Of Geology \University Of Baghdad.

Mineralogical analysis using X-Ray diffraction (XRD):
Three bulk samples of gypcrete where tested using X-Ray diffraction (XRD) instrument in the laboratory of Iraqi geological survey.

Chemical analysis
Three gypcrete soil samples were analyzed to determining major oxides in ALS Laboratory Group\Spain CERTIFICATE SV13090390.The Analytical Procedures showed below in Tablea3 and the analytical result showed in Tables 4and 5.  Results and Discussion: The (XRD) Fig (1,2 and 3) showed that samples at different depth contain gypsum which was identified as the main mineral in gypcrete followed by calcite and quartz as secondary minerals and showed the difference in mineral composition with depth because the mineralogical composition by XRD is not sensitive to concentrations less than 5% in the samples, also we realized that the identification of minerals depends on their crystallinty [3,7].So this instrument couldn't be useful for complete mineral identification (figure 1, 2and3).Microscopic slide for the samples taken (depths 0-0.5m, 0.5-1m and 1-1.5 Table .1)showed that gypsum was the dominated material in all samples near earth surface with percentage 63% of soil content, this value number decreases with depth increasing up to 1-1.5m until reaching 8.84% then comes calcite (carb-onate) concentration to be the second dominant and its concentration increases with depth [3] reaching 44% of soil content at 1.5m depth, then there is the Quartz which is 4.9% at the surface and increases to 13.2% at 1.5m depth [11].
All other elements are in between of the above in percentage with some variation, and in some become equal in their percentages in all depths this could be due to the fact that gypsum present in the area is of secondary origin, and has rivals been transported by the action of surface water then redeposit at different long time.This is why it is found concentrated near the surface decreasing within depth (0.5-1.5m).[11,6].
In Table (4,5) show representative samples taken from the surface down to 0.5m.
Silicon oxide is the less, increasing with depth (1-1.5m) up to 33.4%, this fact is repeated for Al, Fe, Na, Mg, K, Ti and C oxides, while Ca, Sr and S oxides decrease with depth as well as L.O.I, Ba oxides remains as it is.These results confirm the fact that gypsum is concentrated at and near surface, while soil increases with depth composed of Na, Si, Fe, Al, Mg..etc.Oxsides [3].Element analysis and their concentration vary within the depth(Table5) showed that some of them increases with depth increasing except Hg, Bi, and As remain without change.Others showed no significant variations [9].This could mean that samples at and near the surface were washed repetitively, this operation caused migration of most of the oxides to the lower parts of the soil, and also there is no ground water effect that can dissolve and carry them again up to the surface in order to reprecipitate them at the surface again.