​​Describing mineral transparency

Diaphaneity, to give its technical term, defines the degree of transparency or opacity of a mineral. As such, it obviously depends on the chemistry and mineral structure of the mineral and thus provides some assistance to mineral identification. 

​In defining diaphaneity, however, we usually refer to only three categories of transparency: opaque, translucent and opaque, each of which inevitable  contains a large number of minerals. Many minerals also may vary in their diaphaneity depending on the form the mineral takes and the conditions in  which it is observed. While many thick specimens of minerals are likely to be somewhat opaque, a thin section of most minerals, viewed under a microscope,  may be transparent. 

This emphasises the importance, even with a property as simple as transparency, to view the mineral under standardised conditions, These include selecting a  representative but reasonably thin specimen (e.g. a sliver from large crystals), and a steady and bright light source (preferably using a light table, or where this isn't available, a bright, white halogen or LED light). It's also useful to rotate the specimen as you view it to see whether the transparency varies between different crystal faces or orientations.

The figure below illustrates the classes of diaphaneity, the minerals that occur in the various classes and the gradational nature of the classification.

Issues with diaphaneity

Diaphaneity is in many ways the simplest property of minerals to describe, but the diagnostic value of the results is limited. The three categories we commonly use to measure diaphaneity - transparent, translucent, opaque - are so broad that each includes a large number of minerals so that knowing what class it's in doesn't much narrow down the choices in terms of identity.

As an indication of the problem one study examined the diaphaneity of over 3500 common minerals. This showed that, while 923 (26%) are defined as transparent, 897 (nearly 26%) as opaque, and 362 (10%) as translucent, many fell into intermediary classes. These included 801 defined as transparent to translucent and a further 144 as translucent to opaque. This still left almost 11 per cent to be defined yet more transitional ways, including 65 classified as subtranslucent-opaque, 36 as transparent to subtranslucent and 29 as transparent to translucent to opaque.

​As can therefore be seen, the classes we use to describe transparency are broad, vague and indeterminate and are only of limited help in mineral identification. So don;'t agonise too much!