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Lithosphere vs Asthenosphere Plate Motion Plate Tectonics and Rock Cycle Geological Processes for Gemstones Formation Magmatic mineral deposits concentrated in igneous rocks; Hydrothermal/Pegmatitic mineral deposits form in association with hot water- or gas-rich fluids Metamoprhogenic mineral deposits concentrated by metamorhism / metasomatism Sedimentary mineral deposits are precipitated from a solution, typically sea water; Placer minerals are sorted and distributed by flow of water (or ice); Residual mineral deposits formed by weathering reactions at the earth's Schematic model of a fully grown kimberlite pipe Pegmatite Pegmatite Gneiss Gneiss Gem-bearing Pegmatites Pegmatite is often found in veins (dikes) penetrating into surrounding continental country rock, although it can exist as very large bodies also (stock or batholith size). As with all felsic rocks, is found on continents. It is one of the last fractionation products and typically results from partial melting of the lower crust. http://geollab .jmu.edu/Fichter/IgnRx/GranAlk4B1.html Pegmatite containing tourmaline Cassedanne, J.P. and Roditi. M., 1996. The location, geology and mineralogy of gem tourmalines in Brazil. Journal of Gemmology, Volume 25, No. 4, pp. 263298. Tourmaline-bearing pegmatites, which commonly are lens-shaped, are variable in their strike and dip, and show a range of patterns of zoning and albitization. Wallrocks are mica schists, quartzites or rarely granite or gneiss, all Precambrian in age. The pegmatites occur either singly or in fields or clusters and belong to the Brazilian tectonic cycle (650-450 m.y.). Five types of gem-bearing pegmatites are described: 1) poorly differentiated, almost homogeneous, with small albitic replacement bodies; 2) poorly differentiated with strong albitization; 3) zoned with small albitic replacement bodies; 4) zoned with strong albitization; 5) lithium-rich, very well zoned, with strong albitization. Spessartite in pegmatite matrix http://www.mineralminers.com/html/spemins.stm This photograph shows giant lithium crystals of the mineral spodumene in a pegmatite in the Black Hills of South Dakota. http://www.science.uwaterloo.ca/earth/geoscience/pegmat.html Spodumene From the Greek, spodoumenos, meaning "burnt to ash". Spodumene forms as flattened, prism-shaped crystals that can be very large, up to 16 meters long and weighing several tons. The crystals have fine, vertical ridges ( s t r i a t i o n s ). Spodumene can be colored white, gray, yellowish, green (hiddenite), and pink to purple (kunzite). http://www.sdnhm.org/fieldguide/minerals Spodumene are an important source of lithium. Hiddenite and kunzite are used as gemstones even though spodumene cracks easily. It is found only in granite pegmatites. It can be found in Scotland, Sweden, Brazil, Burma, and parts of North A m e r i c a. Kunzite, the pale pink to lilac gem variety of the mineral spodumene, is named as a tribute to George Kunz, the legendary gem scholar, gemologist, and gem buyer. It is often found in association with morganite and pink tourmaline, the other popular pink gemstones. Today most kunzite is mined in B r a z i l , A f g h a n i s t a n , a n d M a d a g a s c a r. Hiddenite is the green variety of spodumene. It is an attractive gem mineral, but is rare and for the most part k n o w n o n l y t o c o l l e c t o r s. Gemstones in Volcanic Rocks Peridot in Vesicular Basalt Hydrothermal Gemstone Colombian Beryl Deposits Gemstones as Products of Weathering Origin of Gem Gravels Gemstones in dikes or veins Channel bed rifles Underwater potholes Water Fall Point Bar Tributary Beach Placer Placer Gem Deposits NAMIBIA NAMIBIA COLOMBIA VIETNAM METAMORPHIC GEMSTONE : JADEITE Jadeite - is a sodium aluminum pyroxene that is characterized by its presence in metamorphic rocks formed at relatively high pressure. It can form by a reaction of Albite to produce : NaAlSi3O8 = NaAlSi2O6 + SiO2 Albite = Jadeite + Quartz JET JEWELLERY (Malm) (Dogger) (Lias) http://www.whitbyjet.co.uk/aboutjet.html Monkey-puzzle tree or Chilean pine Amber: the Jurassic Gem Millions of people learned from the movie that amber, which is fossilized pine tree sap, is ancient and valuable, like an a n t i q u e f r o m p r e v i o u s h i s t o r y. The two main sources of amber on the market today are the B a l t i c s t a t e s a n d t h e D o m i n i c a n R e p u b l i c. Amber from the Baltic states is older, and therefore preferred on the market, but amber from the Dominican Republic is more likely to have insect inclusions. Prices of amber can r a n g e f r o m $20 t o $40,000 o r m o r e. www.gemstone.org The amber, which was from the Lower Cretaceous period, was mined in the mountains of Lebanon south of Beirut by Aftim Acra, who has a collection of amber pieces containing 700 insects, including termites, moths, caterpillars, spiders, p s e u d o s c o r p i o n s , a n d m i d g e s , w h i c h d o s u c k b l o o d. Lapis Lazuli is a rock composed chiefly of the blue silicate mineral lazurite, together with calcite and brassy-coloured pyrite which are abundant in the poorer quality material. The vivid blue of lazurite is cause by the sulphur that forms an essential part of its chemistry. Today lapis lazuli is also mined in Siberia and in Chile, but material from these sources usually contains much calcite. Lapis Lazuli Lapis Lazuli is normally a mixture of three minerals: • lazurite (Na,Ca)8 (AlSiO4 )6 (S,SO4 ,Cl)1-2 • calcite (calcium carbonate, which is white) • pyrite (an iron sulfide that is white-gold in color) Lapis lazuli, is a contact metamorphic rock with variable composition and varying physical properties. It usually forms by contact metamorphism of limestones. Lazurite is a felspathoid (somewhat feldspar-like) cubic (thus, isotropic) hardness: 5.5 S.G. 2.7 - 2.9 vitreous / greasy luster. For many centuries the only known deposits were those at Sar-i-Sang, 1 cm.. in a remote mountain valley in Badakhshan, Afghanistan. At Sar-iSang the lapis lazuli occurs as a zone of lenses and veins within white marble, and grades from deep to pale blue with some violet and greenish tints. From here it was exported to the ancient civilizations of Egypt and Sumer (Ira), and later traded throughout the East and into Europe. These mines are still Lazurite crystal in quartz matrix producing the finest quality lapis l a z u l i.