Tuesday, 14 January 2014

A more attractive product of volcanic action.....

Just back from a week enjoying the sunshine of Lanzarote, one of the Canary Islands, part of the modern Spanish provence of La Palmas. 
Lanzarote was called Titeroigatra meaning The Red Mountains by the indigenous people of the Canary Isles, the Guanches, but renamed after the Geonese Navigator Lancelotto Malocello to Insula de Lanzerote Marcelus in the 14th century.
The island of Lanzerote created from Larva
 
The Canary Isles were created by the solidifying of the larva from fiery eruptions following the break up of the African and American continental plates 15 million years ago.
Timanfaya National Park is the volcanic area on Lanzarote and the most recent volcanic landscapes; created here when in September 1730 a fissure eruption started. This lasted for 6 years and buried 11 villages and 200 sq km of agricultural land land was covered and the lava poured into the sea along 20km of coastline.
The volcanos here lie away from an active plate boundary so it is thought that an intermittent supply of lava comes from a hot spot or mantle plume.
So now what about this caught my attention? Well one of the products of all this volcanic activity is the formation of Olivine the mineral variety of the gemstone Peridot.
Olivine is formed in a magma which is rich in iron and magnesium called mafic magma. When the magma cools and as the olivine minerals have a high crystallisation temperature, they are one of the first to form. Some ultramafic rocks can be composed of almost all olivine and these are called dunites or peridotites.
Dunite is a rock type that is almost wholly composed of olivine. 
Olivine is actually a name for a series of minerals which are categorised between the two end members fayalite or forsterite. Fayalite is the iron rich member, forsterite (named after the German naturalist, John Forester) is the magnesium rich member. The two minerals form a series where the iron and magnesium levels vary. Fayalite due to its iron content has a higher index of refraction, is heavier and has a darker colour than forsterite.
Olivine is one of the most common minerals in the earth and is a major rock forming mineral. Despite this, good specimens and large crystals are rare and sought after, small and microscopic grains are found worldwide. Olivine is also found in meteorites. Olivines are susceptible to weathering and can be altered by a series of chemical reactions to form the mineral serpentine.
Olivine has several industrial uses. It is used as a flux for steel production and is also an important ore of the metal magnesium. Read here for more uses.
Olivine's gemstone variety, known as peridot, it is the birthstone of August and is a green-yellow colour. Its chemical formula is given by: (Mg,Fe)2SiO4. Mohs Hardness 6.5 to 7.
It is quite a brittle stone so not one to wear whilst gardening, save it for the special occasions. 
Peridot and diamond ring handmade by Maker Mends Ltd.

Most peridot is actually the magnesium rich forsterite and its colour is caused by the presence of iron ions. Fayalite's higher iron content make for darker, less attractive specimens that are not generally used as gemstones. Peridot is one of the few gemstones that occur in only one colour, “idiochromatic” as their colour is created by the basic chemical composition of the mineral, it can only be found in shades of green. The most valued colour is a dark olive-green.The best coloured peridot has an iron percentage less than 15% and includes nickel and chromium as trace elements that may also contribute to the best peridot colour.
It is a relatively inexpensive gemstone in small sizes, but the value goes up with stones over 5 carats, with 10-15 carat stones very rare and expensive.
The largest cut Peridot weighing 310 carats is on display in the Smithsonian 
National Museum of Natural History, Washington D.C. This was found on as St. John Island in the Red Sea. Peridot was mined for over 3,500 years on the volcanic island of St. Johns Island (also known as Zagbargad; Zabargad; Zebirget; Seberged; Topazios) and the deposits rediscovered in 1900. It is also found in Myanmar (Burma), Arizona, New Mexico, and Hawaii, in the US, China, Pakistan, Norway, Brazil, Australia, Kenya, Mexico, Sri Lanka, South Africa and Tanzania. The United States was one of the largest producers of peridot with an estimated value of production of $1.5 million in 1999.
In Naturalis Historia, Pliny the Elder tells of the first specimen of Peridot being presented to queen Berenice; Theban queen of Lower Egypt, about 300 BC.
In 2006, olivine was found in comet dust brought back from the
 Stardust space probe.
Many of the gift shops sell souvenirs made from the black basalt with olivine crystals.
Clock with a few strategically positioned Olivine crystals glued on. 
Olivine is abundant on Lanzerote and often forms easily noticeable green phenocrysts  (relatively large and usually conspicuous crystal) in black basalt. 
But of course I wanted to find my own samples of olivine, it wasn't that  easy! I found a few but not the hoard I was hoping for. (pride prevented me from purchasing samples from the local mineral shops). 
Black basalt containing olivine phenocrysts as found by me.
If you can't make it to Lanzarote anytime soon the Natural History Museum London has samples of Olivine and Peridot in its mineral and gemstone display.
Uncut Peridot sample on display at the Natural History Museum