b'GarnetFeatureGarnet: The colourful silicate, a speciality mineralgarnet. Rare uvarovite, occurring as tiny green crystals, and the greenish garnetoid hydrogrossular are often added to the main five types, see Table 1. Garnets are found in all colours except blue; some can be colourless, though commonly they are a dark reddish brown. Colour in garnets results from traces of transition metals such as chromium, iron, manganese, vanadium, and titanium. Deer et al (1992) provide an excellent mineralogical review of garnets.Table 1.Garnet groups; chemical formulae, densities and magnetic susceptibilities.Group Species Compositions Density Mag k Don Emerson(g/cm 3) SI x 10 5systemsnsw@gmail.comPyralspites pyrope Mg 3 Al 2 (SiO 4 ) 3 3.58 0Introduction almandine Fe 3 Al 2 (SiO 4 ) 3 4.32 369spessartine Mn 3 Al 2 (SiO 4 ) 3 4.19 474Non-metallic silicate minerals, and silica, are dominant in the earths crust, but do not receive as much published attentionUgrandites grossular Ca 3 Al 2 (SiO 4 ) 3 3.59 0from exploration geoscientists as do the rarer metal and fuelandradite Ca 3 Fe 2 (SiO 4 ) 3 3.86 291resources, and groundwater. However, non-metallics alsouvarovite Ca 3 Cr 2 (SiO 4 ) 3 3.85 (~90?)constitute an important resource. They are indispensable; sand,hydrogrossular Ca 3 Al 2 (SiO 4 ) 3-xOH 4x 3.13.6 0for instance, is the worlds most consumed commodity after[x ~ ]water. Sand, on the whole, is too vast a topic for a short article. Rather, a particularly impressive non-metallic silicate, the garnet,Notes to Table 1:Garnets are nesosilicates comprising stacked SiO 4tetrahedra linked by which can occur as a sand, is the focus here. interstitial cations: A 3 B 2 (SiO 4 ) 3where A may be Ca, Mg, Fe , Mnand B may The name garnet comes from the Latin for pomegranate.be Al, Fe+, Cr+.Pomum granatum ie, seeded fruit. Small common garnets wereHydrogrossular is really a jade-like garnetoid often intergrown with idocrase (a jade-like complex silicate). Uvarovite occurs as tiny, very pretty, green likened to its reddish seeds. Garnets are a common mineralcrystals and is rare. Accordingly, these two types will not be considered mainly found in regional and contact metamorphics, and in thefurther in the text, being only listed here for completeness.detritals shed from them. They are present in some granites andThe ideal (pure) formula, densities, and magnetic volume susceptibilities pegmatites, also in ultramafics ejected volcanically from the(k) are given for the main species. Note that within two broad groups (pyralspites & ugrandites) there are frequent gradations of composition depths of the earth and upper mantle. They have an interestingowing to solid solution miscibility, but less between the groups.history: being prized in human culture as a semi-preciousThe densities are for pure synthetic garnets prepared using high pressure gem, being used (by believers) in crystal therapy, acting astechniques. Pure garnets do not seem to occur much, if at all, in nature so pathfinders in diamond exploration, providing informationthe use of these theoretical densities needs to consider the effect of each on the deep earth and upper mantle1, and forming an actualgarnet molecule on the properties of the mixed types encountered in nature (Anderson & Jobbins, 1990).resource in deposits of the garnet sands much preferred as anThe paramagnetic & magnetic susceptibilities of garnets are a moot point, abrasive (instead of quartz) in many industries. Owing to theirthe values in the above Table have been taken from Bleil & Petersen (1982).colour and sharp crystal forms they are also sought by mineralThe susceptibilities of almandine, spessartine, and andradite will vary collectors, even if the garnets are not of the transparent gemmyin natural garnets owing to solid solution mixing e.g., spessartine with variety. Gem garnets are sought and sold in affordable jewelleryalmandine, almandine with pyrope, andradite with grossular, and other possibilities (as for densities).the world over. Pyrope and grossular are most likely to be diamagnetic, displaying a small negative susceptibility, however, for convenience in plotting they have been Garnets abound in the metamorphic terrain of Broken Hillascribed a value of zero in Figure 5.NSW. Many years ago, in the pub there, after a days field work,Uvarovites susceptibilities does not seem to have been documented. Limited my friend Thirsty the geologist opined Mate! Garnets aretests I have carried out on unsatisfactory material suggests ~ 90 x 105 SI.great, they got grunt, they grate, and you can trouser a fewSpessartines mag k seems high to me, but I cannot check it as I cannot acquire a pure spessartine. However, many Mn compounds have well dollars by flogging any good ones you find! The recollection ofestablished high paramagnetic susceptibilities e.g., rhodochrosite, MnCO 3 , Thirstys earthy wisdom inspired this article, written, as usual,480 x 105 SIdiscursively, and subjectively.Garnets The five main garnets are variably dense (3.64.3 g/cc, see Figure 1), hard (Mohs hardness7 ), variably paramagnetic Of the twenty or so recognised garnets only five are important(up to ~500 x 105 SI), insulating (virtual dielectrics), moderate here: pyrope, almandine, spessartine (the pyralspites); grossularin thermal conductivity (~3 to 7 W/mC), high in Pwave velocity and andradite (the calcic ugrandites). Almandine is the common(~8500 m/s), and generally inert (resist alteration). Garnets lack cleavage and fracture (sub) conchoidally or unevenly. 1Crystallisation is in the cubic system usually as dodecahedra Petrologists find garnets useful in lower crust and upper mantle research.(12 rhomb faces) or trapezohedra (24 trapezoid faces). Figure2 The detailed nature of pyrope found in ejected eclogitic rock can be veryshows some dodecahedral garnets collected in the field. informative as to pressure and temperature conditions at depth. Pyrope from eclogite inclusions in a basic breccia pipe at Delegate NSW indicatedFigure3 shows small translucent gemmy almandine garnets cut crystallisation in 7-15 kbar, 7001200C range (Deer et al, 1992). and polished into various shapes popular in the gem industry. DECEMBER 2021 PREVIEW 64'