MOUNT SHADWELL AEGIRINE-AUGITE CRYSTALS
The Australian Journal of
Mineralogy published an article on the Aegirine-augite crystals at Mt
Shadwell in its June 2008 (Vol. 14, No. 1) edition. The article had
contributions from Bill Birch, Alan Wood, Allan White, Stuart Mills and
Ross Freeman. We have been given permission to include the article on
this website following communications with Dr Bill Birch of Museum
Victoria.
William D. Birch Geoscience Section, Museum Victoria, GPO Box 666, Melbourne, Victoria, 3001, Australia
Alan Wood 30 Boorook Street, Mortlake, Victoria, 3772, Australia
Allan J. R. White 51 Cox Street, Port Fairy, Victoria, 3284, Australia
Stuart J. Mills Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada, V6T 1Z4.
Ross Freeman Sietronics Pty Ltd, PO Box 3066, Belconnen, ACT 2616, Australia
Abstract
Free-standing aegirine-augite crystals occur in basaltic scoria cones representing the volcanic eruption points of Mt Shadwell and Mt Anakie, on the Cainozoic volcanic plains of Western Victoria, Australia. The crystals are found encrusting white quartz xenoliths in the scoria. They are pale yellow to orange, prismatic in habit and up to 1 cm long. Microprobe analyses show a range in compositions, but all plot within the field of Ca−Na pyroxenes. Optical properties and unit-cell data for crystals from Mt Shadwell are presented. Residual volcanic glass found encrusting the quartz and aegirine-augite crystals at Mt Shadwell has compositions in the quartz trachyte − peralkaline rhyolite range, suggesting extreme fractionation has occurred on a small scale as the quartz reacted with the surrounding basaltic scoria melt after eruption had taken place.
Introduction
Prominent basaltic scoria cones are a characteristic landscape feature on the Cainozoic volcanic plains of the Western District of Victoria. As a group, they generally represent the youngest phase of eruption in any volcanic episode. Many of the scoria volcanoes are important sites for megacrysts and mantle and crustal xenoliths which have enabled both magma generation and crustal structure
beneath the region to be modelled. The ready availability of megacrysts and xenoliths in a number of quarries on scoria cones has, up until recently, overshadowed the presence of a suite of high-temperature minerals which have crystallised in the scoria post eruption. At two sites in particular, Mt Shadwell and Mt Anakie, mineral collectors have been obtaining a range of well-crystallised, mostly microscopic species (e.g. pseudobrookite, potassian nepheline) which
so far have not been studied in detail.
Amongst the most interesting and eye-catching of the discoveries are pale yellow to orange, prismatic crystals encrusting fragments of white reef quartz enclosed in scoria. These have been collected from both Mt Shadwell and Mt Anakie, with specimens from the two sites being so similar
in appearance as to be almost indistinguishable. Interest in the crystals first arose during an excursion to the Mt Shadwell quarry by participants in the Fourth International Mineralogy and Museums Conference held in Melbourne in December 2000. One of the authors (AW) handed a specimen to Professor Jose Coutinho, from Brazil, who was unable to provide a precise identification. Subsequent
examination of more material by two authors (AJRW and RF) led to a tentative identification of esseneite, based on X-ray diffraction and optical examination. The same provisional identification was made for similar crystals collected at Mt Anakie by Judy Rowe in 2002. However, microprobe analysis has now identified the crystals from both localities as aegirine-augite but with slightly different
compositions.
This paper provides descriptions of the aegirine-augite crystals and presents microprobe and optical data supporting the identification. Additional data on the composition of the Mt Shadwell scoria and of residual volcanic glass associated with the crystals enable late-stage chemical trends to be defined.
Geological setting
The two occurrences are some 135 km apart, with Mt Shadwell situated 2 km north of Mortlake and Mt Anakie about 30 km north of Geelong (Figure 1). Mt Shadwell is the highest (135 m relief) of a group of scoria cones that overlie a small tuff deposit and are surrounded by lava flows. As revealed in the large operating quarry (Figure 2), the scoria is generally coarse, both red and black, and
contains abundant volcanic ‘bombs’ containing ultramafic xenoliths, as well as clinopyroxene and anorthoclase megacrysts (Wass and Irvine, 1976; Rosengren, 1994).
Mt Anakie is actually a complex of three composite scoria cones and a maar which are aligned southeast−northwest over a distance of 5 km. The cones are a prominent feature on the skyline, with the northern cone being 170 m above the surrounding plain. They are better known as The
Anakies, with the eastern cone referred to as ‘Eastern Hill’. Substantial quarries operate on the eastern and central cones (Figure 3) and have yielded a range of xenoliths and megacrysts (Wass and Irvine, 1976; Rosengren, 1994).
Figure 1: Map showing the locations of Mt Anakie and Mt Shadwell on the Western District Volcanic Plain.
The (simplified) extent of the lava flows is indicated by the shading.
Figure 2: Mt Shadwell scoria quarry, February 2008. Photo: B. Birch.
Figure 3: Mt Anakie (Eastern Hill) scoria quarry. Photo: Judy Rowe.
Occurrence, Appearance and Optical Properties
At Mt Shadwell, encrustations and crystals of aegirineaugite up to 1 cm long occur at or near the margins of rounded white xenoliths up to 10 cm across enclosed within scoria (Figure 4). The
xenoliths consist predominantly of recrystallised quartz with small amounts of tridymite and
cristobalite. They are considered to be pyrometamorphosed quartz pieces carried up in the magma from the Palaeozoic basement rocks. Many of the inclusions have been fragmented, rounded and etched, suggesting they have undergone resorption in the magma. This resorption may have
continued after eruption, as in many samples there are gaps between the inclusion and the surrounding scoria, creating space in which aegirine-augite crystals have been free to grow. The aegirine-augite crystals have grown directly on the quartz and on the surfaces of the surrounding cavity (Figure 5). They may be accompanied by magnetite, late-forming calcite balls and tiny aragonite needles. In some cavities the quartz fragments are partially coated by transparent, pale yellowish brown glass containing bubbles (Figure 6).
The Anakies occurrence is very similar (Figures 7, 8), although the quartz xenoliths and the aegirine-augite crystals are not as large as those from Mt Shadwell.
Figure 4: Quartz inclusion in Mt Shadwell scoria, showing crust of orange-yellow aegirine-augite crystals; inclusion is 3 cm across. Photo: Lucinda Gibson. Specimen: Museum Victoria, M50822.
Figure 5: Aegirine-augite crystals on quartz inclusion, Mt Shadwell. Field of view is 6 mm by 4 mm. Photo: Ben Heally/Bill Birch. Specimen: Museum Victoria,M50828.
Figure 6: Brown glass on quartz inclusion in scoria from Mt Shadwell. Field of view is 7 mm across. Photo: Lucinda Gibson/Bill Birch. Specimen: Museum Victoria, M50821.
Figure 7: Quartz inclusion with aegirine-augite crystals in scoria from Mt Anakie. Inclusion is 2 cm across. Photo: John Haupt. Specimen: Museum Victoria, M47985.
Figure 8: Aegirine-augite crystals on quartz from Mt Anakie. Field of view is 7 mm across. Photo: Ben Heally/Bill Birch. Specimen: Museum Victoria, M47985.
Figure 9: Thin section photomicrograph in plane-polarised light showing aegirine-augite crystals (yellow−brown colours). Note the pale brown residual glass showing between the crystals. Voids
and silica minerals (quartz, minor tridymite and cristobalite) are white. Image is 3.2 mm across. Photo: Alan White.
In hand specimen and as seen under the microscope, the aegirine-augite crystals from both
localities are prismatic and transparent, varying from pale yellow through orange−yellow to
brownish yellow, rarely to dark greenish. Colour zonation is observable in some crystals, with
colourless to pale yellow cores overgrown by brighter yellow rims. In thin section, the
aegirine-augite crystals show a strong brownish yellow colour, which tends to mask the high
birefringence (Figure 9). Pleochroism varies from lemon yellow (α and γ) to very pale yellow (β). Refractive indices determined for Mt Shadwell crystals gave α=1.707, γ = 1.735, with orientation
Y=b, Z∧c = 30° and 2Vmeas.~ 70°, and with strong inclined dispersion r<v.
X-ray diffraction
Crystals from both localities were examined by routine X-ray powder diffraction at Museum Victoria using a Philips X’Pert diffractometer for an initial identification. The resulting patterns were characteristic of clinopyroxenes in general and no precise identification could be made from them without chemical data. Two separate crystal concentrates from Mt Shadwell were run on a Bruker
D4 X-ray diffractometer at Sietronics Pty Ltd in Canberra, and the d-spacings of the peaks with I/Io greater than 40 were used to calculate approximate unit cell dimensions of a = 9.69, b = 8.83 and
c = 5.38 Å. More accurate cell parameters for aegirine-augite from The Anakies were obtained from
determination of the crystal structure (Mills and Groat, 2008) (see accompanying paper).
Chemical composition
Microprobe analyses were undertaken on representative crystals from both localities, using a
Cameca SX50 instrument under operating conditions of 15 kV and 25 nÅ. The results (see Table 1) show a range of Na-rich, low-Ti, Al-absent compositions. Total iron was partitioned into Fe2O3 and FeO using the method of Droop (1987) and employing an online spreadsheet established by Robert
Sturm (Institute of Physics and Biophysics, Salzburg, Austria). Formula calculation shows that all analyses can be expressed in terms of the aegirine (NaFe3+) endmember and a Ca−Mg−Fe2+ (quad) component. In the Q−J diagram, all analyses plot in the Na−Ca compositional field for pyroxenes (Figure 10). The Anakies crystals as a group contain more of the aegirine endmember than those from
Mt Shadwell, but further sampling and analyses may close the apparent gap. The term aegirine-augite is applicable to all analyses shown, under the pyroxene nomenclature scheme of Morimoto et al. (1988). The respective formulae for the average composition from The Anakies and Mt Shadwell, based on all available analytical data, are:
(Ca0.57,Na0.40)Σ0.97(Mg0.57,Fe3+0.40,Fe2+0.05,Mn0.01,Ti0.01)Σ1.04(Si1.97,Fe3+0.03)Σ2.00O6
(Ca0.76,Na0.24)Σ1.00(Mg0.71,Fe3+0.26,Fe2+0.02,Mn0.01,Ti0.01)Σ1.01(Si1.97,Fe3+0.03)Σ2.00O6
Figure 12: Total alkali versus silica plot for the Mt Shadwell scoria and residual glass. Arrow
shows fractionation extent and direction resulting from reaction between the scoria melt with a
basanite composition and quartz xenoliths, accompanied by aegirine-augite crystallisation.
Formation
Textural relationships suggest that the aegirine-augite crystals at both localities have crystallised under hightemperature conditions during post-eruptive cooling within the scoria mound. Magmatic resorption of the quartz xenoliths, around which the crystals have nucleated, may have commenced prior to eruption, but further reaction, possibly enhanced by the presence of volcanic gases, appears
to have occurred following eruption. This created the space for the crystals to grow between the xenoliths and the enclosing scoria.
Evidence for continuing reaction may be found from the composition of the glass occurring with aegirine-augite crystals on some quartz fragments from Mt Shadwell. It is likely that this glass represents the most fractionated remnant of the original magma, following reaction of the
scoria melt with the quartz, accompanied by crystallisation of the aegirine-augite. Microprobe
analyses (Table 2) show the glass to be rich in SiO2 (67−73 wt %), K2O and Na2O, with a CIPW normative content made up of roughly equal proportions of quartz, orthoclase and aegirine (‘acmite’)
(Figure 11). Mt Shadwell scoria is alkaline in composition (Na2O + K2O of 5−6 wt %) and is classified as basanite (Irving, 1971), with normative nepheline contents of 9−10 wt % (see Table 3). The scoria melt composition is therefore markedly different to that of the glass in contact with the quartz xenoliths, showing that very strong fractionation effects have taken place over short distances. Elementby- element comparison between the two indicate that the combination of reaction with the quartz and crystallisation of the aegirine-augite has led to strong enrichment in Si and K and strong depletion in Al, Mg and Ca in the residual melt fraction. In effect, a basanite melt has been changed to one which could potentially crystallise as a quartz−orthoclase−aegirine assemblage, such as may
occur in a quartz trachyte or peralkaline rhyolite (see Figure 12).
Clinopyroxene crystals of similar appearance to the Victorian aegirine-augites occur in the scoria cones of the Eifel region in Germany (Hentschel, 1987), but these examples have not been studied in detail (Uwe Kolitsch, personal communication), so that comparisons are not possible at this stage.
References
Droop, G. T. R., 1987: A general equation for estimating Fe3+ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria.
Mineralogical Magazine, 51, 431−435.
Hentschel, G., 1987: Die Mineralien der Eifelvulkane. Second, extended edition, Chr. Weise Verlag, Munich, Germany, 177 pp. (in German).
Irving, A. J., 1971: Geochemical and high pressure experimental studies of xenoliths, megacrysts and basalts from southeastern Australia. PhD thesis (unpublished), Australian National University, Canberra.
Mills, S. J. and Groat, L. A., 2008: The crystal structure of yellow aegirine-augite from Mount Anakie, Victoria. Australian Journal of Mineralogy, 14(1), 43-45.
Morimoto, N., 1988: Nomenclature of Pyroxenes. Canadian Mineralogist, 27, 143−156.
Rosengren, N., 1994: Eruption points of the Newer Volcanics province of Victoria: an inventory and evaluation of scientific significance. National Trust of Australia (Victoria) and the Geological Society of Australia (Victoria Division), 387 pp.
Wass, S. Y. and Irvine, A. J., 1976: XENMEG: a catalogue of xenoliths and megacrysts in volcanic rocks of eastern Australia. The Australian Museum, Sydney.
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Aegirine-augite crystals in scoria from
Mt Shadwell and Mt Anakie, Victoria, Australia.
Mt Shadwell and Mt Anakie, Victoria, Australia.
William D. Birch Geoscience Section, Museum Victoria, GPO Box 666, Melbourne, Victoria, 3001, Australia
Alan Wood 30 Boorook Street, Mortlake, Victoria, 3772, Australia
Allan J. R. White 51 Cox Street, Port Fairy, Victoria, 3284, Australia
Stuart J. Mills Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada, V6T 1Z4.
Ross Freeman Sietronics Pty Ltd, PO Box 3066, Belconnen, ACT 2616, Australia
Abstract
Free-standing aegirine-augite crystals occur in basaltic scoria cones representing the volcanic eruption points of Mt Shadwell and Mt Anakie, on the Cainozoic volcanic plains of Western Victoria, Australia. The crystals are found encrusting white quartz xenoliths in the scoria. They are pale yellow to orange, prismatic in habit and up to 1 cm long. Microprobe analyses show a range in compositions, but all plot within the field of Ca−Na pyroxenes. Optical properties and unit-cell data for crystals from Mt Shadwell are presented. Residual volcanic glass found encrusting the quartz and aegirine-augite crystals at Mt Shadwell has compositions in the quartz trachyte − peralkaline rhyolite range, suggesting extreme fractionation has occurred on a small scale as the quartz reacted with the surrounding basaltic scoria melt after eruption had taken place.
Introduction
Prominent basaltic scoria cones are a characteristic landscape feature on the Cainozoic volcanic plains of the Western District of Victoria. As a group, they generally represent the youngest phase of eruption in any volcanic episode. Many of the scoria volcanoes are important sites for megacrysts and mantle and crustal xenoliths which have enabled both magma generation and crustal structure
beneath the region to be modelled. The ready availability of megacrysts and xenoliths in a number of quarries on scoria cones has, up until recently, overshadowed the presence of a suite of high-temperature minerals which have crystallised in the scoria post eruption. At two sites in particular, Mt Shadwell and Mt Anakie, mineral collectors have been obtaining a range of well-crystallised, mostly microscopic species (e.g. pseudobrookite, potassian nepheline) which
so far have not been studied in detail.
Amongst the most interesting and eye-catching of the discoveries are pale yellow to orange, prismatic crystals encrusting fragments of white reef quartz enclosed in scoria. These have been collected from both Mt Shadwell and Mt Anakie, with specimens from the two sites being so similar
in appearance as to be almost indistinguishable. Interest in the crystals first arose during an excursion to the Mt Shadwell quarry by participants in the Fourth International Mineralogy and Museums Conference held in Melbourne in December 2000. One of the authors (AW) handed a specimen to Professor Jose Coutinho, from Brazil, who was unable to provide a precise identification. Subsequent
examination of more material by two authors (AJRW and RF) led to a tentative identification of esseneite, based on X-ray diffraction and optical examination. The same provisional identification was made for similar crystals collected at Mt Anakie by Judy Rowe in 2002. However, microprobe analysis has now identified the crystals from both localities as aegirine-augite but with slightly different
compositions.
This paper provides descriptions of the aegirine-augite crystals and presents microprobe and optical data supporting the identification. Additional data on the composition of the Mt Shadwell scoria and of residual volcanic glass associated with the crystals enable late-stage chemical trends to be defined.
Geological setting
The two occurrences are some 135 km apart, with Mt Shadwell situated 2 km north of Mortlake and Mt Anakie about 30 km north of Geelong (Figure 1). Mt Shadwell is the highest (135 m relief) of a group of scoria cones that overlie a small tuff deposit and are surrounded by lava flows. As revealed in the large operating quarry (Figure 2), the scoria is generally coarse, both red and black, and
contains abundant volcanic ‘bombs’ containing ultramafic xenoliths, as well as clinopyroxene and anorthoclase megacrysts (Wass and Irvine, 1976; Rosengren, 1994).
Mt Anakie is actually a complex of three composite scoria cones and a maar which are aligned southeast−northwest over a distance of 5 km. The cones are a prominent feature on the skyline, with the northern cone being 170 m above the surrounding plain. They are better known as The
Anakies, with the eastern cone referred to as ‘Eastern Hill’. Substantial quarries operate on the eastern and central cones (Figure 3) and have yielded a range of xenoliths and megacrysts (Wass and Irvine, 1976; Rosengren, 1994).
Figure 1: Map showing the locations of Mt Anakie and Mt Shadwell on the Western District Volcanic Plain.
The (simplified) extent of the lava flows is indicated by the shading.
Figure 2: Mt Shadwell scoria quarry, February 2008. Photo: B. Birch.
 |
Figure 3: Mt Anakie (Eastern Hill) scoria quarry. Photo: Judy Rowe.
Occurrence, Appearance and Optical Properties
At Mt Shadwell, encrustations and crystals of aegirineaugite up to 1 cm long occur at or near the margins of rounded white xenoliths up to 10 cm across enclosed within scoria (Figure 4). The
xenoliths consist predominantly of recrystallised quartz with small amounts of tridymite and
cristobalite. They are considered to be pyrometamorphosed quartz pieces carried up in the magma from the Palaeozoic basement rocks. Many of the inclusions have been fragmented, rounded and etched, suggesting they have undergone resorption in the magma. This resorption may have
continued after eruption, as in many samples there are gaps between the inclusion and the surrounding scoria, creating space in which aegirine-augite crystals have been free to grow. The aegirine-augite crystals have grown directly on the quartz and on the surfaces of the surrounding cavity (Figure 5). They may be accompanied by magnetite, late-forming calcite balls and tiny aragonite needles. In some cavities the quartz fragments are partially coated by transparent, pale yellowish brown glass containing bubbles (Figure 6).
The Anakies occurrence is very similar (Figures 7, 8), although the quartz xenoliths and the aegirine-augite crystals are not as large as those from Mt Shadwell.
Figure 4: Quartz inclusion in Mt Shadwell scoria, showing crust of orange-yellow aegirine-augite crystals; inclusion is 3 cm across. Photo: Lucinda Gibson. Specimen: Museum Victoria, M50822.
Figure 5: Aegirine-augite crystals on quartz inclusion, Mt Shadwell. Field of view is 6 mm by 4 mm. Photo: Ben Heally/Bill Birch. Specimen: Museum Victoria,M50828.
Figure 6: Brown glass on quartz inclusion in scoria from Mt Shadwell. Field of view is 7 mm across. Photo: Lucinda Gibson/Bill Birch. Specimen: Museum Victoria, M50821.
Figure 7: Quartz inclusion with aegirine-augite crystals in scoria from Mt Anakie. Inclusion is 2 cm across. Photo: John Haupt. Specimen: Museum Victoria, M47985.
Figure 8: Aegirine-augite crystals on quartz from Mt Anakie. Field of view is 7 mm across. Photo: Ben Heally/Bill Birch. Specimen: Museum Victoria, M47985.
Figure 9: Thin section photomicrograph in plane-polarised light showing aegirine-augite crystals (yellow−brown colours). Note the pale brown residual glass showing between the crystals. Voids
and silica minerals (quartz, minor tridymite and cristobalite) are white. Image is 3.2 mm across. Photo: Alan White.
In hand specimen and as seen under the microscope, the aegirine-augite crystals from both
localities are prismatic and transparent, varying from pale yellow through orange−yellow to
brownish yellow, rarely to dark greenish. Colour zonation is observable in some crystals, with
colourless to pale yellow cores overgrown by brighter yellow rims. In thin section, the
aegirine-augite crystals show a strong brownish yellow colour, which tends to mask the high
birefringence (Figure 9). Pleochroism varies from lemon yellow (α and γ) to very pale yellow (β). Refractive indices determined for Mt Shadwell crystals gave α=1.707, γ = 1.735, with orientation
Y=b, Z∧c = 30° and 2Vmeas.~ 70°, and with strong inclined dispersion r<v.
X-ray diffraction
Crystals from both localities were examined by routine X-ray powder diffraction at Museum Victoria using a Philips X’Pert diffractometer for an initial identification. The resulting patterns were characteristic of clinopyroxenes in general and no precise identification could be made from them without chemical data. Two separate crystal concentrates from Mt Shadwell were run on a Bruker
D4 X-ray diffractometer at Sietronics Pty Ltd in Canberra, and the d-spacings of the peaks with I/Io greater than 40 were used to calculate approximate unit cell dimensions of a = 9.69, b = 8.83 and
c = 5.38 Å. More accurate cell parameters for aegirine-augite from The Anakies were obtained from
determination of the crystal structure (Mills and Groat, 2008) (see accompanying paper).
Chemical composition
Microprobe analyses were undertaken on representative crystals from both localities, using a
Cameca SX50 instrument under operating conditions of 15 kV and 25 nÅ. The results (see Table 1) show a range of Na-rich, low-Ti, Al-absent compositions. Total iron was partitioned into Fe2O3 and FeO using the method of Droop (1987) and employing an online spreadsheet established by Robert
Sturm (Institute of Physics and Biophysics, Salzburg, Austria). Formula calculation shows that all analyses can be expressed in terms of the aegirine (NaFe3+) endmember and a Ca−Mg−Fe2+ (quad) component. In the Q−J diagram, all analyses plot in the Na−Ca compositional field for pyroxenes (Figure 10). The Anakies crystals as a group contain more of the aegirine endmember than those from
Mt Shadwell, but further sampling and analyses may close the apparent gap. The term aegirine-augite is applicable to all analyses shown, under the pyroxene nomenclature scheme of Morimoto et al. (1988). The respective formulae for the average composition from The Anakies and Mt Shadwell, based on all available analytical data, are:
(Ca0.57,Na0.40)Σ0.97(Mg0.57,Fe3+0.40,Fe2+0.05,Mn0.01,Ti0.01)Σ1.04(Si1.97,Fe3+0.03)Σ2.00O6
(Ca0.76,Na0.24)Σ1.00(Mg0.71,Fe3+0.26,Fe2+0.02,Mn0.01,Ti0.01)Σ1.01(Si1.97,Fe3+0.03)Σ2.00O6
Figure 12: Total alkali versus silica plot for the Mt Shadwell scoria and residual glass. Arrow
shows fractionation extent and direction resulting from reaction between the scoria melt with a
basanite composition and quartz xenoliths, accompanied by aegirine-augite crystallisation.
Formation
Textural relationships suggest that the aegirine-augite crystals at both localities have crystallised under hightemperature conditions during post-eruptive cooling within the scoria mound. Magmatic resorption of the quartz xenoliths, around which the crystals have nucleated, may have commenced prior to eruption, but further reaction, possibly enhanced by the presence of volcanic gases, appears
to have occurred following eruption. This created the space for the crystals to grow between the xenoliths and the enclosing scoria.
Evidence for continuing reaction may be found from the composition of the glass occurring with aegirine-augite crystals on some quartz fragments from Mt Shadwell. It is likely that this glass represents the most fractionated remnant of the original magma, following reaction of the
scoria melt with the quartz, accompanied by crystallisation of the aegirine-augite. Microprobe
analyses (Table 2) show the glass to be rich in SiO2 (67−73 wt %), K2O and Na2O, with a CIPW normative content made up of roughly equal proportions of quartz, orthoclase and aegirine (‘acmite’)
(Figure 11). Mt Shadwell scoria is alkaline in composition (Na2O + K2O of 5−6 wt %) and is classified as basanite (Irving, 1971), with normative nepheline contents of 9−10 wt % (see Table 3). The scoria melt composition is therefore markedly different to that of the glass in contact with the quartz xenoliths, showing that very strong fractionation effects have taken place over short distances. Elementby- element comparison between the two indicate that the combination of reaction with the quartz and crystallisation of the aegirine-augite has led to strong enrichment in Si and K and strong depletion in Al, Mg and Ca in the residual melt fraction. In effect, a basanite melt has been changed to one which could potentially crystallise as a quartz−orthoclase−aegirine assemblage, such as may
occur in a quartz trachyte or peralkaline rhyolite (see Figure 12).
Clinopyroxene crystals of similar appearance to the Victorian aegirine-augites occur in the scoria cones of the Eifel region in Germany (Hentschel, 1987), but these examples have not been studied in detail (Uwe Kolitsch, personal communication), so that comparisons are not possible at this stage.
References
Droop, G. T. R., 1987: A general equation for estimating Fe3+ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria.
Mineralogical Magazine, 51, 431−435.
Hentschel, G., 1987: Die Mineralien der Eifelvulkane. Second, extended edition, Chr. Weise Verlag, Munich, Germany, 177 pp. (in German).
Irving, A. J., 1971: Geochemical and high pressure experimental studies of xenoliths, megacrysts and basalts from southeastern Australia. PhD thesis (unpublished), Australian National University, Canberra.
Mills, S. J. and Groat, L. A., 2008: The crystal structure of yellow aegirine-augite from Mount Anakie, Victoria. Australian Journal of Mineralogy, 14(1), 43-45.
Morimoto, N., 1988: Nomenclature of Pyroxenes. Canadian Mineralogist, 27, 143−156.
Rosengren, N., 1994: Eruption points of the Newer Volcanics province of Victoria: an inventory and evaluation of scientific significance. National Trust of Australia (Victoria) and the Geological Society of Australia (Victoria Division), 387 pp.
Wass, S. Y. and Irvine, A. J., 1976: XENMEG: a catalogue of xenoliths and megacrysts in volcanic rocks of eastern Australia. The Australian Museum, Sydney.
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