An introduction to minerals and rocks under the microscope
An introduction to minerals and rocks under the microscope

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An introduction to minerals and rocks under the microscope

1.3.3 Lustre

The term 'lustre' refers to the surface appearance of a mineral, which depends on the way it reflects light. Typical terms used to describe a mineral's lustre include vitreous (rather like glass), metallic and resinous. Quartz (Figure 5a) has a vitreous lustre, as do many other silicate minerals, such as feldspar. When transparent, like window glass or clear coloured glass, the term 'glassy' lustre may be used instead of vitreous: quartz, for example, often has a glassy lustre.

Some opaque minerals scatter light very strongly, giving rise to shiny, reflective surfaces and a metallic lustre, such as seen in pyrite, galena (both available in the Digital Kit [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] ) and magnetite (Figure 9a). Other examples are pearly lustre (looking like pearls) (Figure 9b), silky lustre (like shiny threads or fibres) (Figure 9c), and a dull or earthy lustre (Figure 9d). Note that, as in the case of gypsum (Figures 9b and c), different varieties of the same mineral may show different types of lustre.

Figure 9 Some examples of lustre: (a) metallic (magnetite) (specimen 4 cm across); (b) pearly (gypsum, variety selenite) (field of view 7 cm across); (c) silky (gypsum, variety satin spar) (5.5 cm); (d) dull or earthy (psilomelane, a form of manganese oxide) (field of view 4.5 cm across).
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