These veins deposited in rock fissures form the matrix for precious metals i.e., gold. Gold is
therefore mainly found in quarts veins. It can therefore be expected that mining of gold will lead
towards significant release of quarts. In South African mines the rock generally contains 60 – 90%
SiO2 in the crystalline form of quarts Ref.(2).
Alpha (low) quartz is the type of silica that is most often released during mining, blasting and
construction activities. Cristobalite is associated with ceramic, refractory and diatomaceous earth
industries. Cristobalite can also be formed due to processing of crude materials that involve
heating to high temperatures. It is therefore important to note that silica quartz can be transformed
during industrial processes, especially if heated, and this can change the crystalline structure
(Figure 2).
Figure 2: Influence and cooling on the crystalline structure of quarts
The transformed crystalline structure is more pathogenic or toxic than the original alpha quartz
crystalline structure. If the heating process does not exceed 578°C, beta (high) quartz are formed.
It will, however, revert to its original structural and physical property if cooled. If the heating
process exceeds 578°C but are lower than 1 470°C tridymite will form. Temperatures beyond 1
470°C will results into the formation of cristabolite. Neither of the last two transformed structures
can revert to alpha quarts. The smelting point of transformed quarts is about 1 710°C. This is the
primary reason why sand is used as moulds because its smelting point is higher than most of the
metals that are usually used for casting processes Ref.(7).
Factors that influence toxicity and potential to induce
fibrosis by crystalline silica
Quartz, cristobalite and tridymate have the highest potential to introduce fibrosis in the lungs. The
biological reactivity of the three types of crystalline silica is not similar. Quartz potential to induce
fibrosis is higher than tridymite and tridymiteʼs potential to induce fibrosis is higher than cristobalite
Ref.(8). Freshly fractured quartz proved also to have an increased potential to induce a fibrotic
reaction in lung tissue compared to that of “aged” quartz. The presence of radicals on the fracture
surface is the primary determinant in terms of toxicity. SiOH groups on the surface of crystalline
silica are capable to form hydrogen bonds with membrane components. The hydrogen bond
2
Heated to 573
0
C
Cooled
Beta (high) quartz
Heat > 14700 C
Tridymite Cristobalite
Heat to 8700 14700 C
Different crystal structure
and physical properties
Alpha (low) quartz