Aggregates - Geomaterials: Under the Microscope - page 66

148

variously to cataclasite ( 148 ), ultracataclasite ( 149 ), and
ultramylonite ( 150 ). These rocks contain higher
proportions of disordered quartz and are likely to be
alkali-reactive (Wigum, 1996).
Quartzite and chert (and flint) are important rock types
when considering aggregate for concrete, as they are both
commonly used in concrete and are potentially alkali-
reactive. Classification of these rocks can be challenging
and with this in mind, Hunt (1995) has devised an
engineering classification of quartzite and chert to aid
alkali-silica reactivity assessment. The classification is
based on texture, fabric, and grain size definitions as
shown in Table 16 . Using this classification, Figure 151
shows diaquartzite, Figure 152 shows metaquartzite, Figure
153 shows cherty quartzite, and Figure 77 shows chert.
AAR is discussed further on pp. 109-114.

148 Granite cataclasite with a porphyritic texture with
porphyroclasts of feldspar (centre) set in a groundmass
largely composed of finely crystalline quartz; XPT,
×35.

149 150

149 Ultracataclasite consisting of an unfoliated
groundmass of finely crystalline to cryptocrystalline
silica, with rare strained porphyroclasts; XPT, ×35.

150 Ultramylonite with a foliated groundmass of
finely crystalline to cryptocrystalline silica, with rare
strained porphyroclasts; XPT, ×35.

Table 16 Classification of the quartzite group of rocks (from Hunt, 1995)

TEXTURE

0%
5%
50%
95%
100%

(Ortho)quartzite Diaquartzite Metaquartzite

Cherty quartzite Cherty diaquartz Cherty metaquartz

Quartzitic chert Quartzitic diachert Quartzitic metachert

Chert Diachert Metachert

Original grain No original grain structure, Interlocking crystals,

structure visible interlocking crystals visible restructuring

INCREASING MATURITY