Bushveld aureole metapelite outcrop  Outcrop of metamorphosed Timeball Hill Formation, sampling locality for M32. Bushveld aureole metapelite outcrop  Detail of metamorphosed sedimentary layering. Darker layers originally more clay-rich, paler layers originally of silt or fine sand. Banding exists on several scales, and is preserved down to sub-millimetre scale. Bushveld aureole metapelite outcrop detail  Nature of mineral banding: more Al-rich layers are packed with blocky porphyroblasts of staurolite, andalusite and biotite. More silty layers have sparser, smaller biotite and staurolite. Bushveld aureole metapelite outcrop  Outcrop of metamorphosed Timeball Hill Formation, sampling locality for DLB9. Grey nodular porphyroblasts are andalusite. Quarry outcrop of andalusite hornfels  Grey graphitic metapelite with randomly oriented long andalusite prisms. Hoogenoeg quarry, photo by Dan Lovegrove.
Porphyroblastic andalusite hornfels  Pale andalusite on bedding-parallel surfaces of metapelitic hornfels at Longsight, sampling locality for BM1 - BM4 Bushveld aureole metapelite outcrop  Closer to contact, approximately on sillimanite isograd, smaller-sized andalusite porphyroblasts in typical layers. Bushveld aureole metapelite outcrop  Sillimanite hornfels, close to igneous contact, cut by irregular partial-melt veins. Bushveld aureole metapelite outcrop  Fine-grained hornfels close to igneous contact, with spherical sillimanite nodules. Bushveld aureole petrography: M32  Scan of entire thin section. Large grey andalusite; dark yellow inclusion-packed staurolite; brown biotite; fine-grained matrix of muscovite, plagioclase, quartz.
Bushveld aureole petrography: BM1  Graphitic metapelite from Longsight, whole-slide scan. Andalusite (chiastolite habit) and cordierite (hexagonal outlines) concentrated in graphitic mica-rich layers. Biotite porphyroblasts present throughout. Bushveld aureole petrography: BM3  Graphitic metapelite from Longsight, whole-slide scan, showing banding of different types and scales. Porphyroblasts of biotite, cordierite (partly replaced by biotite) and andalusite. Bushveld aureole petrography: BM8  Higher-grade metapelite from the Silverton Formation, whole-slide scan. Turbid rims on small andalusite porphyroblasts contain fibrolitic sillimanite. Matrix contains granoblastic cordierite with biotite, plagioclase and quartz. Bushveld aureole petrography: BM8  Detail from whole-slide scan, showing cluster of andalusites with somewhat rounded morphology associated with fibrous sillimanite and biotite. Waters & Lovegrove microstructures  General view from a thin-section scan of sample M32. Dark inclusion-clouded staurolite, grey andalusite, brown biotite. Matrix is very fine muscovite, plagioclase, quartz. Regular-shaped clear patches in matrix (and also in andalusite, staurolite), some cored by biotite, are pseudomorphs after chloritoid and (locally) cordierite. View is 1.5 x 1 cm.
Waters & Lovegrove microstructures  M32 matrix view with staurolite, biotite, and rectangular domains pseudomorphing chloritoid. Field of view 3.5 mm. Waters & Lovegrove microstructures  M32 matrix view with staurolite, biotite, and rectangular domains pseudomorphing chloritoid. Crossed polars, field of view 3.5 mm Waters & Lovegrove microstructures  M32 andalusite poikiloblast with rectangular inclusion-free domains commonly cored by biotite. In other samples these can be shown to be originally chloritoid tablets. Andalusite growth has impinged upon and partly enclosed adjacent biotite and staurolite poikiloblasts. Field of view 7 mm. Waters & Lovegrove microstructures  M32 andalusite poikiloblast as previous image, in crossed polars. Field of view 7 mm. Waters & Lovegrove microstructures  DLB-215B, low-grade metapelite from deeper levels in the Timeball Hill Formation, showing the typical habit of chloritoid in chloritoid-chlorite-muscovite 'slate'. This rock is the lower-grade compositional equivalent of samples like M32 and DLB-9B. Field of view 0.7 mm.
Waters & Lovegrove microstructures  DLB-9B, pseudomorphs after chloritoid. Matrix region with tabular-shaped areas of quartz containing narrow aligned mica flakes, and a few possible high-relief relics of chloritoid. Field of view 0.8 mm. Waters & Lovegrove microstructures  DLA-9A, pseudomorph after chloritoid. From Annesley quarry, slightly lower grade than DLB-9B. This tablet is mainly composed of quartz and biotite, but contains numerous small slivers of relict chloritoid. Field of view 1 mm. Waters & Lovegrove microstructures  DLB-9B. Backscattered electron image of rectangular pseudomorph composed of coarser-grained quartz with mica flakes. Surrounding matrix consists of random finer muscovite with fine-grained plagioclase (light grey) and quartz (dark grey). Waters & Lovegrove microstructures  DLB-9B, pseudomorphs after chloritoid, in core of large andalusite poikiloblast, where they form domains with fewer quartz inclusions. Field of view 1.2 mm. Waters & Lovegrove microstructures  DLB-9B, biotite inclusion, and pseudomorphs after chloritoid, in core of large andalusite poikiloblast, crossed polars. Field of view 0.6 mm.
Waters & Lovegrove microstructures  DLB-9B, pseudomorphs after chloritoid, in core of large andalusite poikiloblast. These examples appear to show the 'hourglass' zoning of inclusion density commonly seen in chloritoid, preserved during the replacement by andalusite. Field of view 0.4 mm. Waters & Lovegrove microstructures  DLB-9B. Quartz-rich pseudomorph after chloritoid wrapped by outer part of staurolite poikiloblast. Field of view 1 mm. MetaGeol Cld pseudomorph slide Waters & Lovegrove microstructures  DLA-9A (Annesley mine). A chloritoid pseudomorph, aligned NW-SE, is partly overgrown by staurolite and andalusite. MetaGeol DLA-9A interp slide
Waters & Lovegrove microstructures  M32. Backscattered electron image showing biotite overgrown by staurolite. The outline of the biotite and its pattern of quartz inclusions appear to be faithfully preserved by the staurolite. So also is the distribution of matrix plagioclase grains and muscovite flakes overgrown by the upper part of the staurolite. Scale bar is 0.5 mm. Waters & Lovegrove microstructures  DLB-9B. Outer parts of staurolite poikiloblasts commonly encroach upon biotite (inclusion-poor domain completes the shape of the external biotite flake). These replacements are not found in staurolite cores, where instead lath-shaped pseudomorphs after chloritoid are typical (right). This implies that some staurolite growth predated the appearance of biotite. Field of view 1.2 mm. Waters & Lovegrove microstructures  M32. Staurolite replacing adjacent biotite grains, with the original outline of the biotites clearly visible. Filed of view 3.5 mm. Waters & Lovegrove microstructures  DLB-9B. Lath-shaped domains marking former chloritoid, in staurolite (centre) which has been enveloped by andalusite. The central lath shape passes into the andalusite (below), and its upper end is continuous with a quartz-rich pseudomorph which forms a small embayment in the staurolite. Width of view 1 mm. Waters & Lovegrove microstructures  Staurolite growing in the matrix region. Note the inclusion-free rim zone around much of the perimeter. This might be expected if staurolite continues to grow right up to the thermal peak, when heating rates slow right down. Note also that chloritoid shapes occur in the core of staurolite, but quartz pseudomorphs after chloritoid project into the outline. Width of view 2.5 mm.
Waters & Lovegrove microstructures  A pair of staurolite crystals partly enclosed by andalusite (right), but projecting into matrix (left). The inclusion-poor outer rims are much more strongly developed on the matrix side of the poikiloblasts. The inference is that staurolite growth probably outlasted that of andalusite. Width of view 2.5 mm. Waters & Lovegrove microstructures  DLB-9B. Garnet overgrowing matrix and pre-existing biotite. Former extent of biotite grain is marked by change from small rounded quartz inclusions to small aligned opaque platelets with scattered larger quartz inclusions. Plane-polarised light. Width of view 1.2 mm. Waters & Lovegrove microstructures  DLB-9B. Hexagonal pseudomorph of quartz, biotite and muscovite after cordierite. Cordierite porphyroblasts of identical habit are seen in more Mg-rich samples from the overlying Boven Shale. Width of view 1 mm. Waters & Lovegrove microstructures  DLB-9B. Pseudomorphed cordierite partly enclosed by staurolite. Field of view 1.4 mm. Waters & Lovegrove microstructures  DLB-9B. Pseudomorphed cordierite composed of relatively coarse quartz grains, partly enclosed by staurolite. Field of view 1.4 mm.
Waters & Lovegrove microstructures  Backscattered electron image showing small oval cordierite relics (bright) within quartz (dark) in pseudomorph. Width of view 0.2 mm. Waters & Lovegrove microstructures  X-ray composition map for Al showing pseudomorph after cordierite wholly enclosed by andalusite (bright spongy material). The dominant phase in the pseudomorph is muscovite (medium grey) with some biotite (dark grey) and minor quartz (black). Width of view 1.4 mm. Waters & Lovegrove microstructures  Hexagonal cordierite in BM3, graphitic Boven Shale member, Longsight, for comparison with pseudomorphs in M32 and DLB-9B. Waters & Lovegrove microstructures  Hexagonal cordierite in BM3, graphitic Boven Shale member, Longsight, for comparison with pseudomorphs in M32 and DLB-9B. Outer part has been replaced by aggregate of quartz, feldspar and mica, coarser than the surrounding matrix. Microstructures in graphitic metapelites  BM3, Boven Shale Member, Longsight. Andalusite (chiastolite habit) in graphitic metapelite.
Microstructures in graphitic metapelites  BM3, Boven Shale Member, Longsight. Andalusite (chiastolite habit) in graphitic metapelite. Crossed polars. Microstructures in graphitic metapelites  BM1, Boven Shale Member, Longsight. Sedimentary layering, biotite poikiloblasts in silty layers, Square section of andalusite, hexagonal outline of (altered) cordierite in graphitic shaly layer. Field of view about 2 cm across. Microstructures in graphitic metapelites  BM3, Boven Shale Member, Longsight. Concentration of graphite around margins of small biotite porphyroblasts and corner of largely inclusion-free andalusite. Field of view c. 2.5 mm across. Microstructures in graphitic metapelites  BM3, Boven Shale Member, Longsight. Graphitic 'cleavage dome' above growth face of andalusite porphyroblast. Field of view c. 1 mm across. Microstructures in graphitic metapelites  BM3, Boven Shale Member, Longsight. Broader view of part of andalusite porphyroblast showing graphitic 'cleavage domes' at growth faces. Note that biotite inclusions in andalusite have trapped the graphite accumulation and allowed subsequent andalusite growth to enclose matrix quartz grains until prevented by further graphite build-up. Field of view c. 2.5 mm across.
Microstructures in graphitic metapelites  DLB-383b, Timeball Hill Fm, Annesley. Core detail of andalusite porphyroblast in graphitic hornfels. Note (1) pink core, implying sector zoning with Mn concentration into (001) growth faces; (2) small rounded Qz inclusions along arms of the chiastolite cross; (3) sparse tubular Qz 'inclusons' at right angles to external (110) growth faces. Field of view 1 mm. Microstructures in graphitic metapelites  BM3, Boven Shale Member, Longsight. Cordierite porphyroblasts, partly replaced by Qz, Pl and mica. Note regrowth of skeletal biotite on clouded cores that represent grains originally included in the cordierite. Field of view 2.5 mm. Microstructures in graphitic metapelites  BM3, Boven Shale Member, Longsight. Closer detail of biotite inclusions in partly-replaced cordierite porphyroblasts, showing skeletal overgrowths on clouded original cores. Field of view 1 mm. Microstructures in high-grade metapelites  BM-8, upper part of Silverton Fm, near Burgersfort. Sample from andalusite-sillimanite transition, with small andalusite porphyroblasts surrounded by fibrolite in matrix of Bt, Crd, Pl and Qz. PPL, field of view ~4 mm. Microstructures in high-grade metapelites  BM-6, upper part of Silverton Fm, near Burgersfort. Sample from andalusite-sillimanite transition. Small andalusite porphyroblasts with a little fibrolite in matrix of Bt, Crd, Pl and Qz, with a few late muscovite flakes (bright colours). Crossed polars, field of view 2 mm.