January 15, 2023
Geology Word of the Week: L is for Luster
Posted by Evelyn Mervine
A sample of iron-rich rock with metallic hematite (gray in color, this variety of hematite is called “specular hematite”) and dull hematite (reddish brown in color). Picture by myself.
def. Luster (or Lustre if you use British spelling):
The way in which the surface of a mineral or rock interacts with light. Words used by geologists to describe luster include metallic, sub-metallic, dull (or earthy), vitreous, waxy, silky, greasy, pearly, and adamantine.
Luster is a physical property that is used by geologists to help identify minerals and rocks. Other physical properties that geologists use for identification are hardness, density, magnetism, color, streak (the color of the mark a rock or mineral leaves on a streak plate), cleavage, and crystal form.
When using luster to identify a mineral, it is important to know that many minerals can have more than one type of luster, in the same way that many minerals come in different colors. Thus, luster should be used cautiously for mineral identification. A good example is shown in the photograph at the top of this post. This photograph shows a shiny rock with a metallic luster. At first glance, you might think that the rock contains mica, which can have a similar metallic luster and a similar color. However, the rock is actually an iron-rich rock that mostly consists of hematite. The portion of the rock with the metallic luster is specularite, also known as specular hematite. Note that a portion of the rock has a reddish brown color. This part of the rock has a dull or earthy luster and is also hematite. To properly identify the minerals in this rock, a geologist must use other properties in addition to luster. For example, the density of the rock (iron is heavy) and the streak of the rock. Specular hematite will leave a reddish brown streak while mica will leave a whiteish streak.
The photograph below shows another example of luster. In this case, there are quartz crystals with a vitreous (or glassy) luster.
Quartz crystals with vitreous luster. Picture by myself.
The photograph below shows anther example of luster. This rock contains serpentine and talc. The white talc has a pearly or perhaps waxy luster. The green serpentine has a silky luster.
A rock containing talc and serpentine. The white talc has a pearly luster while the green serpentine has a silky luster. Picture by myself.
That’s all for this week’s word… stay tuned for next week!
