b'GarnetFeatureTable 2.Garnet physical propertiesOther minerals that may be confused with or passed off as natural garnets include: red-orange spinel MgAl 2 O 4(3.60 g/cm3, k = 0, H8); synthetic orange corundum Al 2 O 3(4.0 g/cm3, k = 0, H = 9); synthetic reddish garnet YAG Y 3 Al 5 O 12(4.55g/cm3, k = 0; H = 8.25); red-brown zircon ZrSiO 4( 4.7g/cm3,k = 0, H7.5); and coloured glass doped for higher densities (2.34.5 g/cm3, if no Fe or Mn k = 0, H6). [H is Mohs hardness, which for garnets is7.5]See also Mavko et al (1998)Forty five garnets that I collected or acquired over the years from a wide variety of locations were used in trial measurements of density and susceptibility. The data are given in Table 2 and plotted in Figure 5. The results are informative. Almandines plot along the almandinepyrope trend; the two high density spessartines have the highest susceptibilities (but fall short of the Bleil & Peterseon (1982) values, see Table1); andradites plot, more or less, along around the andraditegrossular trend; and the gems, which when purchased (cheaply) were only labelled as garnet, appear to be of the common almandinepyrope type as would be expected for such material. Simple susceptibilitydensity plots would seem to be quite useful in garnet identification, complementing optical measurements of refractive index and absorption spectra. There is ambiguity in estimating garnets types with low susceptibility and relatively low density i.e. grossular and pyrope. Other tests may be required. Natural pyrope does tend to have a distinctive reddish colour whereas grossular is usually green/yellow, or off white (as in Figure 2 front right). For translucent specimens the absorption spectra, determined with a spectroscope, are different: pyrope absorbs transmitted light in a band around 564 nm wavelength, grossular does not. The five Mexican samples #2731 in the lower left of the k-D plot were confirmed as grossulars by colour, and also by thermal conductivity tests. Grossular is reported as having a higher thermal cond. than red garnet (Clark, 1966).Industrial garnetsGarnet, physically, is a tough stone. Small stones of the appropriate (bloody) colour were used by Asiatic tribes as missiles in slingshots and muzzleloaders, being deemed more deadly than lead. They were so used by the Kashmiri Hanza in 1892 in frontier conflict with the British.Garnet, in our time, has found a niche as a specialty mineral for use in abrasive blasting media, waterjet cutting, abrasive powders, water filtration plants, and other uses. Garnets inertness and resistance to chemical attack account for its use in final stage under-draining beds in water purification. Notes to Table 2: However, most garnet is used in sub mm grainsizes for Samples #135 natural stone crystals mmcm diameter; #3645 smallabrading operations of one sort or another. Table 3 attempts to summarise the characteristics, availability, and cost of dimension cut, faceted gems; sample #1 from Kenya (reputedly pyrope);garnet compared to quartz, which has most of the market. #2, 3 Harts Range NT; #4, 5, 7 Broken Hill NSW; #6 Massachusetts USA; #810 Turkey; #1117 India; #1821 China; #22 Broken Hill NSW; #2324Compared to quartz, garnet is denser, tougher, harder, Brazil; #2526 Mali; #2731 Mexico; #32 Mali; #33 Japan; #34 Hampshireand stronger (see Table 3 velocities, and Youngs (stiffness) Tasmania; #3545 provenances uncertain modulus). It is also safer and recyclable. A key quality is Densities measured by Archimedes principle following Emerson (1990)fracture which, for a good abrasive garnet, results in edges to 3 decimal places for smaller mm size specimens, porosities generally low (0.1 to 1.0% range) and associated with fractures; magnetic volumeself-sharpening with continuing use. Not all garnets have susceptibility (k) measured by change of inductance when samplethis splintering facture necessary for continuing abrasion.inserted into test coil following Yang and Emerson (1997)Four other samples tested thought to be from either Broken Hill NSW orGarnets sands are mined in only a few locations around Harts Range NT, are not cited in Table owing to high magnetic susceptibilitiesthe world. Australia has a mine in mineralised sand dunes arising from included magnetite octahedra up to 2mm in size [data: 4.08, 875;near Geraldton W.A. Garnet is far more valuable than quartz 4.11, 811; 4.10, 2550; 4.15, 5906; gm/cm3 and SI x 105, respectively] and it would seem that there is ample scope in Australia for 67 PREVIEW DECEMBER 2021'