Gahnite
By Bob Farrar
by Bob Farrar
The Mineral of the Month for January is gahnite. Named for Swedish minerologist Johan Gottlieb Gahn, gahnite is often called zinc spinel. However, it is a distinct mineral species and worth learning about on its own. Gahnite is a member of the spinel group. It consists of zinc aluminum oxide, ZnAl2O4. Spinel, in contrast, is magnesium aluminum oxide, MgAl2O4. It crystalizes in the cubic system, typically as octahedral crystals. Gahnite is usually dark blue-green, but can also be yellow or brown. Physical properties include a hardness of 7.5 – 8.0, specific gravity of 4.62, and vitreous luster. Spinel has similar properties, except that is lighter, with a specific gravity of 3.55. Crystal form and hardness distinguish members of the spinel group from most other minerals, with specific gravity distinguishing gahnite from spinel.
Gahnite occurs in metamorphosed zinc ore deposits and pegmatites. Some of the best localities for gahnite are in the northeastern United States. Mt. Apatite in Maine is a wellknown locality. Gahnite has been found at several quarries on Mt. Apatite, including the Pulsifer and Keith Quarries, but it is most abundant at the Hole-in-the-Ground Quarry. Crystals up to 5 cm have been found at the Hole-in-the-Ground Quarry. They are the typical blue-green color, but are usually of somewhat distorted crystal form, and coated with a layer of rusty brown micaceous material that makes them hard to recognize. Another famous New England locality is Charlemont, Massachusetts. Gahnites there are typically cleanly crystalized without any coatings. Crystals of 2 to 3 cm. have been found in the zinc ore deposits of Franklin, New Jersey. Gahnite is also known from Maryland; small crystals have been found at the Ben Murphy Mine in Burtonsville, the Kensington Mica Mine in Silver Spring, and Mineral Hill in Carroll Co. Another regional locality is Louisa Co., Virginia. Worldwide localities include Australia, Brazil, and several European countries. The type locality is Falun, Sweden.
In rare cases, gahnite can be facetted, but gems are typically small. Gem quality material has been found in Maine, Brazil, Nigeria, and Sri Lanka. Gahnite is not of much economic importance, nor is it an important gemstone. However, it is an interesting mineral for collectors. Especially for those who collect in the Mid-Atlantic and Northeast, it is a mineral with which everyone should be familiar. CGMS Auctions Chesapeake has three auctions each year where members may sell hobby related items. Each auction is limited to 10 members, and each member is limited to 10 items. There is also a mandatory call in to Bernie ahead of each auction so we can plan the space. The auction rules and the necessary forms are on our website on the Members "tab". They will be in the February issue of Chippers Chatter. Lynne will discuss all the procedures during the January meeting.
The Mineral of the Month for January is gahnite. Named for Swedish minerologist Johan Gottlieb Gahn, gahnite is often called zinc spinel. However, it is a distinct mineral species and worth learning about on its own. Gahnite is a member of the spinel group. It consists of zinc aluminum oxide, ZnAl2O4. Spinel, in contrast, is magnesium aluminum oxide, MgAl2O4. It crystalizes in the cubic system, typically as octahedral crystals. Gahnite is usually dark blue-green, but can also be yellow or brown. Physical properties include a hardness of 7.5 – 8.0, specific gravity of 4.62, and vitreous luster. Spinel has similar properties, except that is lighter, with a specific gravity of 3.55. Crystal form and hardness distinguish members of the spinel group from most other minerals, with specific gravity distinguishing gahnite from spinel.
Gahnite occurs in metamorphosed zinc ore deposits and pegmatites. Some of the best localities for gahnite are in the northeastern United States. Mt. Apatite in Maine is a wellknown locality. Gahnite has been found at several quarries on Mt. Apatite, including the Pulsifer and Keith Quarries, but it is most abundant at the Hole-in-the-Ground Quarry. Crystals up to 5 cm have been found at the Hole-in-the-Ground Quarry. They are the typical blue-green color, but are usually of somewhat distorted crystal form, and coated with a layer of rusty brown micaceous material that makes them hard to recognize. Another famous New England locality is Charlemont, Massachusetts. Gahnites there are typically cleanly crystalized without any coatings. Crystals of 2 to 3 cm. have been found in the zinc ore deposits of Franklin, New Jersey. Gahnite is also known from Maryland; small crystals have been found at the Ben Murphy Mine in Burtonsville, the Kensington Mica Mine in Silver Spring, and Mineral Hill in Carroll Co. Another regional locality is Louisa Co., Virginia. Worldwide localities include Australia, Brazil, and several European countries. The type locality is Falun, Sweden.
In rare cases, gahnite can be facetted, but gems are typically small. Gem quality material has been found in Maine, Brazil, Nigeria, and Sri Lanka. Gahnite is not of much economic importance, nor is it an important gemstone. However, it is an interesting mineral for collectors. Especially for those who collect in the Mid-Atlantic and Northeast, it is a mineral with which everyone should be familiar. CGMS Auctions Chesapeake has three auctions each year where members may sell hobby related items. Each auction is limited to 10 members, and each member is limited to 10 items. There is also a mandatory call in to Bernie ahead of each auction so we can plan the space. The auction rules and the necessary forms are on our website on the Members "tab". They will be in the February issue of Chippers Chatter. Lynne will discuss all the procedures during the January meeting.
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