1.2.3: Minerals and Mineral Varieties
- Page ID
- 18279
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1.10 Dogtooth spar is named after the crystal shape
1.11 Iceland spar is a clear variety of calcite
An individual mineral species, such as calcite, is defined by its unique chemical and physical properties. All calcite is mostly CaCO3, with atoms arranged in the same way, no matter the size or shape of the sample. However, calcite, like many other minerals, has more than one named variety, based on crystal shape, composition, color, occurrence, or other things. Dogtooth spar – an example is in Figure 1.10 – is a distinctive variety of calcite found in some caves, and Iceland spar (Figure 1.11) is a clear variety typically in cleavable rhomb shapes.
Gemologists commonly name mineral varieties based on color. Figures 1.12, 1.13, and 1.14, below, show three varieties of beryl. Beryl is called aquamarine if it has a light blue color, emerald if it is green, and morganite if it is pink. The different colors stem from very small compositional differences; all beryl is essentially Be3Al2Si6O18. Aquamarine, however, contains small amounts of Fe2+, emerald contains small amounts of chromium (Cr3+) and vanadium (V3+), and morganite contains Mn2+. Even small amounts of these transition metal ions can give minerals strong coloration.
1.12 Aquamarine (beryl)
1.13 Emerald (beryl)
1.14 Morganite (beryl)