Tourmaline group

Cyclosilicates are those compounds with three or more tetrahedra linked in closed, ring-like structures. The ratio between Si and O is 1 3. This group includes the important gem species beryl and the tourmaline group. The 11 members of the tourmaline group display an unusual property known as piezoelectricity. When pressure is applied to a crystal, an electrical charge builds up at either end of it. This makes these minerals useful in pressure gauges and other electrical equipment. Quartz and a few other, less common minerals, also display this property. 

Treasure Lake Group, overlain by felsic ignimbrite sheets of the Faber Group (Goad et al. 2000). Ore minerals mainly consists of Fe-, As-, Co- and Cu-sulphides, native Au and Bi. Intense, pervasive, polyphase iron-oxide (magnetite-dominant)-hornblende-biotite-tourmaline-K-feldspar-carbonate replacive alteration occurs in the upper metasedimentary sequence below the volcanic-sedimentary unconformity... 

Depending on the composition of disseminated and medium-coarse-grained ore, they can be divided into two basic groups sulphides and chloritic tourmaline ores. In the sulphide ore, the minerals are represented by pyrite, pyrrhotite, arsenopyrite, chalcopyrite, galena and stannin. Less common are sphalerite and bismuth. 

In his recent book, Dietrich (1985) described the common habits of tourmaline as being acicular, filiform, asbestiform, and chalcedonylike, and as whiskers. This last term indicates that tourmaline composition fibers have been synthesized. The wide range of forms for minerals in this group probably accounts for some of the early confusion when samples of tourmaline were equated with asbestos. Schorl, now known to be NaFe Al6(B03)3Si60ig(0H)4, usually occurs as dark green or black acicular crystals. Based on their appearance alone, schorl could easily be mistaken for an amphibole. 

The optical spectra of blue tourmalines have attracted considerable attention focused mainly on assignments of Fe2+ —> Fe3+ IVCT bands, positions of crystal field bands for Fe2+ ions expected to be located in two different octahedral sites, and intensification mechanisms of these crystal field bands (e.g., Faye et al., 1968., 1974 Wilkins et al., 1969 Bums, 1972a Smith, 1977, 1978a Mattson and Rossman, 1984,1987b). Curve-resolved spectra yielded two sets of paired bands (Faye et al., 1974). One set at 14,500 cm-1 and 9,500 cm-1 assigned to Fe2+ in the Al or c-site (point group Q mean Al-O = 192.9 pm) yielded A0 and CFSE values of about 11,000 cm-1 and 4,900 cm-1, respectively. The second set of bands at 13,200 cm-1 and 7,900 cm-1 attributed to Fe2+ in the Mg or b-site (point group Cm mean Fe-O = 202.5 pm) provided A0 and CFSE values of approximately 10,000 cm 1 and 4,500 cm-1, respectively. 

In addition to the chemical groups of the Dana system, there are also structural groups of minerals. These describe two or more mineral species that have the same or similar crystalline structure, while differing chemically. Not all species are placed within structural groups. Well-known groups include garnets, tourmalines, feldspars, clays, and micas. 

The tourmalines are a group of 10 mineral species, only two of which are commonly used as gemstones. Names are given to colored tourmaline, regardless of which species the stones truly are. Verdelite is green, indicolite yellow, rubellite pink or red, siberite violet-red, and achroite is colorless. 

Fig. 5.10a Mt. Madison (80°27 S, 160°02 E) is located on the south side of the mouth of the Byrd Glacier. Its north-facing slope contains three small cirques that are separated from each other by two bedrock spurs which are composed of the Shackleton Limestone of the Byrd Group. Fig. 5.10b The crest of the western spur contains an outcrop of pegmatite composed of spodumene, lepidolite, bismuthinite, and tourmaline (Faure and Felder, 1984). The spur on which the pegmatite is located is marked by an x. This map is an excerpt of the topographic map Cape Selborne, Antarctica (SU 56-60/2) published in 1963 by the US Geological Survey in Washington, DC Photo G. Faure...

Piezoelectric materials can be grouped into the class of natural crystals, such as quartz or tourmaline, into one of polymers, such as polyvinylidene fluoride (PVDF) or that of polycrystalline ceramics. 

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