Gemology Hub | Gemstone & Mineral Identification Guides

A–Z Minerals: A — Andalusite. Andalusite is an interesting mineral for both geology students and crystal collectors. Its shape and the rocks around it can tell a useful story. An andalusite specimen. Photo: Moha112100, CC BY-SA 3.0 , via Wikimedia Commons . Image unchanged. What is andalusite? Its formula is Al 2 SiO 5 . Kyanite and sillimanite have the same chemical formula, but their atoms are arranged differently. Minerals like these are called polymorphs. Simple andalusite properties Crystal system: orthorhombic. Hardness: about 6.5–7.5 on the Mohs scale. Specific gravity: about 3.13–3.16. Streak: white. Shine: glassy. Shape: crystals may be long, with a nearly square cross section. Pink, green, gray and other colors occur. A crystal may also contain many inclusions, so transparency varies. The Handbook of Mineralogy gives the mineral data and occurrence details. How does andalusite form? It commonly forms when clay-rich rocks are changed by heat near an intrusion of molten...

A–Z Minerals: A — Albite. A pale crystal beside another mineral may look unimportant. Albite is worth a closer look. It is a common feldspar that helps us understand many rocks. Albite (pericline), Ziller valley, Austria; specimen about 10 × 5 cm. Photo: Didier Descouens, CC BY-SA 4.0 , via Wikimedia Commons . Image unchanged. What is albite? Albite is the sodium-rich end member of the plagioclase feldspar series. Its ideal formula is NaAlSi 3 O 8 . It has a triclinic crystal structure. In simple terms, this describes how its atoms are arranged. Albite properties at a glance Hardness: 6–6.5 on the Mohs scale. Color: commonly white or gray; other pale colors occur. Streak: white. Luster: glassy, sometimes pearly on cleavage surfaces. Specific gravity: about 2.60–2.65. Breakage: brittle, with strong cleavage directions. Where does it form? Albite occurs in granite, pegmatites and several other rocks. It also grows in some mineral veins. Quartz and mica may occur with it. Its name ...