Geological history and the importance of pegmatite veins.

Pegmatite veins are common in almost all the rock type in Ranchi city of India.

by

Dr. Nitish Priyadarshi

Pegmatite, meaning “joined together,” was first applied to graphic granite sometime before 1822. Subsequently, it has been extended to refer to any abnormally coarse grained rock of overall igneous character. The term “pegmatite” was first used by a French mineralogist René Haüy but he used this term as a synonym of graphic granite. Contemporary meaning was given to the rock type in 1845 by an Austrian mineralogist Wilhelm Heidinger.

Most of pegmatites are more than 1 centimeter across; grains up to a meter or two across are relatively common; and individual crystals up to several meters in greatest dimension have been reported. In any case, no matter how large the grains, most pegmatites have typical igneous rock textures.

Pegmatites are associated with plutonic or intrusive rocks and were evidently formed by slow crystallization at considerable depths below the surface.

Individual pegmatite masses may be classified as simple or complex. The simple ones have an overall homogeneity and consist almost wholly of microcline perthite and quartz plus or minus minor amounts of biotite and /or black tourmaline.

The complex pegmatites tend to be compositionally zoned and to contain, along with quartz and microcline, large quantities of clevelandite, noteworthy amounts of muscovite, and well formed crystals of such minerals as apatite, beryl, topaz, colored tourmaline, and spodumene, plus a number of less common minerals that contain elements such as lithium, niobinum, tantalum, cesium, uranium, and the rare earths.

Pegmatites consist of minerals which are found also in the rocks from which they are derived, e.g. granite-pegmatites contain principally quartz and feldspar while gabbro-pegmatites consist of diallage and plagioclase. Rare minerals, however, often occur in these veins in exceptional amount and as very perfect crystals. The minerals of the pegmatites are always those which were last to separate out from the parent rock. As the basic minerals are the first formed the pegmatites contain a larger proportion of the acid or more siliceous components which were of later origin.

Most simple pegmatites occur as dikes within large igneous masses or their surrounding country rocks. Most complex pegmatites occur as lenticular pods or irregularly shaped masses within the country rocks surrounding large masses or as apparently isolated masses within metamorphic rock terranes.

Pegmatites are not rare rocks but their overall volume is small. They form small marginal parts of large magma intrusions known as batholiths. They form as a late-stage magmatic fluid starts to crystallize. This fluid is rich in water, other volatiles, and chemical elements incompatible in main magmatic minerals.

This is the reason why pegmatites are so coarse-grained and why they contain so much unusual minerals. They are coarse-grained because of high volatile content which makes the magma less viscous and therefore enhances mineral growth (chemical elements are free to move to look for and join a suitable and already existing crystal). Unusual minerals form because the fluid is enriched in exotic chemical elements like lithium, boron, beryllium, rare earth elements, etc. These elements are forced to form their own mineral phases because they are rejected by major rock-forming minerals like quartz, feldspar, and others.

Pegmatites are very irregular not only in distribution, width and persistence, but also in composition. The relative abundance of the constituent minerals may differ rapidly and much from point to point. Sometimes they are rich in mica, in enormous crystals for which the rock is mined or quarried (India). Other pegmatites are nearly pure feldspar, while others are locally (especially near their terminations) very full of quartz. They may in fact pass into quartz veins (alaskites) some of which are auriferous.

This wealth of minerals makes pegmatites often valuable as a mineral resource. Pegmatites may be mined because of their high content of feldspars, clay (if weathered), mica, or many metal-bearing minerals. Pegmatite is also a source of gems like beryl, tourmaline, zircon, etc.

Most pegmatites are granites with or without exotic minerals but mafic pegmatites (gabbro, diorite) are known as well. Silica undersaturated (without quartz) magmatic rocks may be also pegmatitic.

Pegmatite intrusions in the rocks in Ranchi city of India.

Above pictures shows the pegmatite intrusions in the rocks of Ranchi city.

The Pegmatite veins are common traversing almost all the rock type of the area. Their width varies from few meters to tens of meters.  Coarse pegmatitic intrusive are quite abundant, though they tend to cluster at places and result in irregular distribution. They are mainly composed of feldspar (with potash feldspars being much more abundant than the plagioclase) and quartz. The other minerals which are present are tourmaline, muscovite and rarely garnet. At places, the pegmatite contains streaks and specks of pyrite.  

Rifle range- This is the highest point of Ranchi, situated in Bariatu, east of Bharamdih hills. The major rock type is granite gneiss with augen shaped crystals of felsic minerals. Large number of intrusions of pegmatites veins is very common in this part . These intrusions are very thick are in the form of sills and dykes, mainly dykes. Pegmatite contain large crystals of tourmaline and garnet. Muscovite is also present in some proportion.

The other hills where the intrusion of pegmatite is seen are Jagnnathpur hill, Tagore hill, Bharamdih hill etc.

Reference:

Dietrich, R.V. and Skinner, B.J. 1979. Rocks and rock minerals. John Wiley & Sons, U.S.A.

Priyadarshi, N. 1998. A handbook geology of Chotanagpur. Aoyushi Publication, Ranchi, India.

http://www.sandatlas.org/2012/09/pegmatite/

Jharkhand state of India can be the treasure trove of gemstones.

Gemstones of Jharkhand State of India.

by

Dr. Nitish Priyadarshi

Jewels and gemstones have provided an irresistible attraction to all human beings from time immemorial. They were considered as symbols of wealth and power and wearing attractive jewellery by men and women can be traced back to prehistoric times.
During historic times gemstones were in use not only as objects of adornment, but also as an investment.
A mineral is defined as a “naturally occurring homogeneous solid with a definite chemical composition and highly ordered atomic structure usually formed by inorganic processes.” Of the three thousand and odd known minerals only about a hundred species are used as gemstones. These minerals are generally characterized by pleasant colour, transparency, high refractive index, superior hardness and often by their unusual properties such as chatoyancy and iridescence. The most important qualities of gemstones are colour, brilliance, durability and rarity.

Several ancient Indian texts have described a number of varieties of gemstones. Probably in ancient days gemstones were used to cater to one’s aesthetic taste. Later, however, gemstones appear to have been used as talisman to bring in better luck and to remove the bad effects of planets positioned in the horoscope. Through the ages certain gemstones have been assigned to “nine major planets” in Hindu astrology. Possibly the sight of celestial objects as seen from the earth prompted the ancient Indians to assign certain stones to certain planets.

Gemstones of Jharkhand:

Tourmaline:
Tourmaline occurs with a full range of colours. Transparent pink-red, blue, green, yellow-brown and uncoloured varieties are used as gemstones. Black opaque stones are also often cut and worn as mourning jewellery. Tourmaline is found in schists and pegmatic rocks with cavities or in alluvial deposits.
In Jharkhand three varieties of tourmaline are found in the Koderma area. They are green tourmaline, black tourmaline and blue tourmaline (Indicolite).

Moonstone:
Moonstone's delicate beauty and its long heritage make it perhaps the most familiar gem quality member of the feldspar group. Moonstone is a variety of orthoclase which has a blue schiller (sheen) caused by the reflection of light from the internal structure of alternate layers of albite and orthoclase feldspars. When light falls between these thin, flat layers, it scatters in many directions producing the phenomenon called adularescence.
In Jharkhand this gemstone having export quality is concentrated in Koderma area.

Amazonite:
Amazonite (sometimes called "Amazon stone") is a green variety of microcline feldspar.
The name is taken from that of the Amazon River, from which certain green stones were formerly obtained, but it is doubtful whether green feldspar occurs in the Amazon area. Because of its bright green colour when polished, amazonite is sometimes cut and used as a gemstone, although it is easily fractured.
For many years, the source of amazonite's color was a mystery. Naturally, many people assumed the color was due to copper because copper compounds often have blue and green colors. More recent studies suggest that the blue-green color results from small quantities of lead and water in the feldspar (Hoffmeister and Rossman, 1985). This mineral is found in the Koderma area of Jharkhand.

Citrine:
Citrine is transparent, coarse-grained variety of the silica mineral quartz . Citrine is a semiprecious gem that is valued for its yellow to brownish color and its resemblance to the rarer topaz. Pale yellow to a madeira orange in all of its glorious golden and yellow colors. The yellow color is from the presence of iron, the darker the color - the higher the grade.Natural citrine is rare compared to amethyst or smoky quartz , both of which are often heated to turn their natural color into that of citrine. The name citrine is derived from the French word citron - lemon. The term citrine is not familiar to most Indian consumers and it is better known to them as golden topaz or quartz topaz.
Koderma area of Jharkhand state is known for citrine deposit.

Hessonite:
Hessonite or Cinnamon Stone is a variety of grossular, a calcium aluminium mineral of the garnet group. It has a characteristic red color, inclining to orange, much like that of gem zircon.
Hessonite is mainly concentrated in Chatra district of Jharkhand.

Chrysoberyl (cat’s eye):
Like the eye of a sleek feline predator, the chrysoberyl cat's eye winks at the astonished observer – a real miracle of Nature! That's why only this attractive gemstone has the right to the short, fitting name of "cat's eye". This gem is really something special with its narrow, bright band of light on a shimmering golden background, which seems to glide magically across the surface when the stone is moved.
From a mineralogical point of view, chrysoberyls are aluminium oxide containing beryllium, and thus actually have little in common with the beryls, which belong to the silicate family. Indeed, with their excellent hardness of 8.5 on the Mohs scale, they are clearly superior to the beryls.
Singhbhum in Jharkhand state is known for the deposits of cat’s eye.

Garnet:
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. Six common species of garnet are recognized based on their chemical composition. They are pyrope, almandine, spessartite, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite.
Garnets species are found in many colors including red, orange, yellow, green, blue, purple, brown, black, pink and colorless. The rarest of these is the blue garnet. It changes color from blue-green in the daylight to purple in incandescent light, as a result of the relatively high amounts of vanadium.
In Jharkhand, Hazaribag district is known for garnet deposits.

Beryl:
Varieties of beryl have been considered gemstones since prehistoric times.Colorless beryl is called goshenite, pink beryl is morganite, red beryl is bixbite or "red emerald" or "scarlet emerald," clear bright yellow beryl is "golden beryl," yellow-green beryl is heliodor, green beryl is emerald, blue beryl is aquamarine, and deep blue beryl is maxixe. Red beryl is extremely rare.
Beryl of various colors is found most commonly in granitic pegmatites, but also occurs in mica schists in Jharkhand. Beryl is often associated with tin and tungsten ore bodies.
Koderma ,Hazaribag and Giridih are famous of its Beryl deposits. In Ranchi district light green and blue beryl are found.


Reference:
Karanth, R.V. 2000. Gems and Gem Industry of India. Geological Society of India, memoir,45. Bangalore.

Hoffmeister and Rossman (1985). " ". Am. Min. 70: 794–804.

Priyadarshi, N., 1998, A Handbook of Geology of Chotanagpur. Aoyushi Publication, Ranchi, India.

http://en.wikipedia.org/wiki/Moonstone_(gemstone)

http://en.wikipedia.org/wiki/Amazonite

http://www.all-that-gifts.com/se/citrine.html