دكتور / حسن بخيت

بوابة العرب للثروات الطبيعية

Gemstones in Egypt

Introduction

 

Some minerals enjoy an unusual beauty that makes them suitable for use as gemstones. This was probably the very first use of minerals by man, and in Egyptian Museum we see the evidence in form of beads and polished stones made into necklace, rings and bracelets presented as offerings to gods and kings.

 

Among the most valued stones used in Egypt, emerald, turquoise and peridot top the list. These were obtained from the Central Eastern Desert, Sinai and Zabargad Island respectively.

 

Currently, non-of these stones are extracted from Egypt, saves for the occasional pick of a broken emerald crystal of a turquoise concretion by the locals in the Eastern Desert or Sinai. This can not be attributed to exhaustion of reserves, since these were never assessed or estimated, but mostly to the lack of systematic exploration and identification of the potential mining sites. It is of interest to note that at present, these are no valid exploration licences for these gemstones, except a licence covering 10 hectars (Lat.23° 36’) and (Long.36ْ 11‛) issued to El Nasr Phosphate Company to search for and exploit peridot but with no actual activity. Similarly, another licence covering an area of about 20 km2 was granted in 1991 for the private sector to prospect for emeralds in Wadi Sikait (Lat.24˚ 26‛ & 24° 45‛) and (Long.34° 42‛ & 34° 53‛).

 

Field activities included the proper identification of rock units hosting the gems, their field relations and structural-tectonic setting and the identification of the geological environment under which these gems were formed.  Samples collected were subjected to mineralogical, petrographic and XRD investigation for the proper identification of the gemstones and their host rocks.

 

       Complete chemical analysis for major, minor and trace elements were carried out, in addition to the determination of certain properties considered very important in the study of gems.

 

An assessment of the occurrences of emerald, peridot and turquoise in Egypt and their economic potentials are tried here. This lead to the way to future exploitation of a valuable commodity.

 

The concerned occurrences are Zabargad Island (peridot), Zabara –Sikait-Umm Kabut velt (emerald) and Sarabit El Khadem (turquoise).  See the location map (fig.1). Table (1.1) gives a listing of the minerals that are commonly used as gemstones as well as their most important physical properties that made them valuable as gem materials.

 

1-Peridot

A-Introduction   

Olivine represents a solid solution series from forsterite (Mg2 SiO4) to faylite (Fe2 SiO4). Peridot is gem variety of olivine and represents an in intermediate number in the olivine series. The name olivine from the Latin oliva (olive) because of its colour.

Peridot is an ancient gem, often referred to as chrysolite from the Greek meaning (yellow stone). Cat-eye and star peridots are known, but are very rare.

The relatively low hardness of peridot means that ring stones will show scratches rather rapidly. Cleavage may allow an occasional stone to split. The color shows a considerable variation in shades depending on the locality of origin. Brown gem is rare, being rich in Fe but can be very beautiful when it is more golden than brown. The best green gemstone has an iron content of about 15-21%. More iron gives unattractive muddy colour. Very bright colour results from trace of Cr. . Olivine crystallizes in the orthorhombic system and forms biaxial crystals usually seriated , corroded grains often occur as rolled pebbles . Streak of minerals is white or pale yellow . Luster is vitreous to oily , and hardness is 6.5-7 . Specific gravity is 3.2-3.4 . Cleavage is imperfect , while fracture is concoidal . Refractive index ranges between 1.63 and 1.69 , birefringence is 0.038 and pleochroism is weak . Peridot may contain inclusions of many types , glass ballslike bubbles . Zabargad peridot can be distinguished from the Burmese by their inclusions .

The zabargad peridot typically contains chromite crystals and abundant fluid inclusions, whereas the Burmese peridot lacks fluid inclusions.

 

B-Location and previous works on Zabargad:

      Gem olivine (pridot) occurs only in one location in Egypt, that is Zabargad Island (or Saint John’s Islnad). It is situated some 54 km SE of Ras Banas, Eastern Desert at lat.23° 36‛ 16″ N and Long.36° 11‛ 24″ E.

        The Island can only reached by boat, as it has airstrip. Mines were exploited by Ancient Egyptians early as 1500 B.C.

Barthoux (1922) gave the flowing analysis for the peridots:

 

 

SiO2

41.0%

MgO

48.9%

FeO

9.0%

MnO+NiO

0.8%

Al2O3

--------

L.O.I

0.15%

Total 100.15

 

 

 

 

The melting point of Egyptian pridot was reported to be 144.

 

C-Geology of Zabargad Island:

Zabargad Island is situated at the intersection of one of the Red Sea transform faults and western edge of the central zone of Red Sea (fig.2.1). Along this transform may many ring complexes are known to occur in the Eastern Desert. The Island is located 54 km SE of Ras Banas. (fig.2.2) gives the generalized geological map of the area.

Peridot occurs in association with peridotites which are in turn associated with minor gabbro and troctolite .Peridotites are exposed as three main masses intruding old metamorphic rocks of Precambrian age and consist of gneisses and amphibolites. The intrusive and cumulate nature of the ultramafic rocks is evident. The three masses are named northern, central and southern. They represent structural highs on the island. The northern and central peridotites are spinel lherzolites while the southern is plagioclase lherzolites and is named the Peridot Hill and is the largest one. It is located along the southwestern shore of the island and is bound by a major fault zone from the north. Along and close by this fault zone the peridotite is strongly serpentinized and hydrothermally altered, with the development of peridot , granierie, cancrinite, septchlorite (Takla and Griffin, 1982).

The intensive ancient workings for peridot are found in such altered and weathered zones. However, peridot occurs as disseminations throughout the peridotites.

D-Petrography

In hand specimen, the peridotites are generally fresh with a dark green color. This becomes yellow brown along fractures as a result of variable degrees of serpentinizations.

The microscopic examination showed that the peridot-bearing rock is a peridotite (harzburgite) composed essentially olivine and lesser hyperthene. The accessory opaque minerals are chromspinel (pictotite), green spinel and magnetite.

E-Mineralogy

Microscopic investigations: the gem quality olivine (peridot) occurs as scattered euhedral to subhedral large colorless crystals in dunite and peridotite. It is clear and contains no inclusions.

The crystals are very fresh, rarely fractured and show slight serpentinization along fractures. The color and absence of iddingsite alteration point out to Mg-rich composition.

The specific gravity of peridot is 3.90.

F-Chemical composition:

1-The peridot composition is Fo93 Fa7

2-Most of the nickel is fixed in peridot

3-Almost all Co is in the peridot structure

 

Table 2.3 Chemical composition of peridot and dunite:

 

                           peridot                                 dunite

 

  Sio2                 38.52                                     34.92

 TiO2                  0.02                                       0.02

 Al2O3                ---                                           ---

 Fe2O3               0.74                                       6.42

 FeO                   7.91                                       3.70

 MgO                  49.50                                     41.21

 CaO                  0.05                                        0.21

 MnO                 0.12                                        0.13

 Na2O               1.48                                        1.22

K2O                   0.59                                       0.42

P2O5                 0.12                                       0.24

L.O.I                  0.34                                       10.58

Total                  99.39                                     99.07

Minor and trace elements (ppm)

Ni                      4900                                       7600

Cr                      70                                           200

Co                     306                                         327

Cu                    220                                          112

Structural formula

2(Mg Fe Mn) 0. (Si Al) 0.

-------------------------------------------------------------------------------------------------------

 

G-Occurrence and genesis:

 

Peridot or gem variety olivine was mined only from the eastern slope of Peridot Hill, in the vicinity of the major fault zone that limits the Hill from north. This is the only area where the pridotite is highly serpentinized and altered. This mod of occurrence led (El Shazly and Saleeb, 1978) to relate the formation of peridot to hydrothermal processes. (Kurat et al, 1982) stated that the gem occurs in open cavities in veins cutting across the highly serpentinized perdotite, few crystals attached to vug walls but most of them are founded in the rubble at the bottom of the vugs.

See many points of view for other authors such as: (Clochiatti et al, 1981;Kurat et al, 1982;Kouvula, 1981;Keller, 1990) concerning to the genesis. Finally it is concluded that peridot is of primary magmatic origin, formed at the earliest stages of crystallization upper mantle. This is supported by the fact that it is Mg-rich (Fo93 Fa9) and its occurrence in the form of phenocrysts in many fingers –grained groundmass. The role played by hydrothermal solutions (probably heated seawater) is serpentinization of the groundmass and loosening of phenocrysts (peridot). This was mainly restricted to fault and shear zone where water percolation was facilitated along fractures, and hence the restriction of peridot occurrences to such zones of fractures.

 

                   Table 1.1 Mineral commonly used as gemstone:

 

Mineral

General composition

Chief colour

Transp.

hardness

Sp.Gr.

R.I

Auzurite

Cu3(CO3)2(OH)2

Deep blue

n.T

3½-4

3.8

high

Beryl:

 

-Aquamarine

 

-Emerald

-Heliodor

-Goshenite

-Morganite

-Common beryl                     

 

Be3Al2S16O18

 

 

 

Blue, bluish green.

Green

Yellow orange

Colorless

Pink, peach

Green, yellowish

green, colorless

 

 

 

 

 

T

 

T

T

T

T

T

 

 

 

 

 

7½-8

 

 

 

 

 

2.6-2.9

 

 

 

 

 

mod.

 

 Chrysoberyl

Alexandrite

 

 

Cymophane

 

Be3Al2O4

 

 

 

 

Green(daylight) red artificial light

 

Yellowish green

 

 

T

 

 

n.T

 

 

 

 

 

 

 

 

 

3.5-3.8

 

 

high

Chrysocolla

Cu2H2Si2O5(OH)4

Green, blue

n.T

2-4

2-2.4

low

Corundum:

-Ruby

-Sapphire

 

Star ruby and star-sapphire

Al2)3

 

 

 

Pink, red, purple.

Colorless, blue, violet.

 

T

T

 

n.T

 

9

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