Almandine

Mineral Name: Almandine

Chemical Composition: Iron Aluminum Silicate, Fe3Al2Si3O12

Color: Deep red, brown/ red, red-violet, black

Streak: colorless

Hardness: 7- 7.5

Cleavage/Fracture: Almandine has no cleavage. When broken, almandine has a rough or irregular surface called an uneven fracture. Almandine can also present a conchoidal fracture. Conchoidal fractures are rounded indentations in a mineral that look like a shell.

Crystal Form: Almandine crystal forms include dodecahedral, a twelve sided polyhedron where all sides of the crystal are equidimensional and trapezohedral, a twenty-four sided polyhedron with equidimensional sides. Sometimes the crystals form into a modified combination of both dodecahedral and trapezohedral forms. Crystals call also have striations on the crystal faces.

Luster: Vitreous

Special Features: Almandine is paramagnetic. When almandine is heated it becomes weakly attracted to magnetic fields or becomes magnetic itself!

Another useful characteristics of garnets is when they form something called a snowball garnet. A snowball garnet shows mineral inclusions that seems to be rotated into the mineral as it was forming, much like one would form the base of a snowman. Snowball garnets that are found in the Appalachian Mountains of Vermont, for example, have the amazingly unique ability to help geologist understand how the Appalachian Mountains were built and determine the rate at which rocks have folded in this area. Growth rate of the garnet crystals can be determined by comparing rubidium isotope decay at the center relative to their margins of the mineral (5). Information retrieved from Appalachian garnets show that as the supercontinent Pangea was forming about 380 mya, Eurasia and Africa were drifting towards North America. Eventually these continents collided into North America and over time caused the rock folding seen in the Appalachians referred to as “nappes” (5). Inside the rock folds, garnets continued to form as the rocks around them contorted, causing the crystals to turn at about 20 to 30 degrees every million years. The rotation of the garnets tells us how fast the rocks around them deformed and, therefore, give geologists a record of plate collision and mountain building spanning millions of years (5).

Varieties: Common Garnet (dark, brownish-red opaque), Precious Garnet (deep red transparent), Syrian Garnet (slightly purple)

Mineral Group: Almandine is part of the the Silicate Group of minerals and more specifically, almandine is a Nesosilicate. A nesosilicate is a silicate containing only single, non-combining groups of tetrahedrons. Almandine is also a part of the Garnet Mineral Group, a group of closely related isomorphous (having same crystal form, but different specific elements) minerals that form a series with each other (2).

Environment: Almandine most commonly forms at convergent plate boundaries where regional metamorphism is happening. Chemical bonds are broken due to high heat and intense pressure caused by subduction/collision, causing the mineral structure to recrystallize in such a way that it is stable in the new temperature-pressure environment. Almandine is often found within a schist matrix that has formed from shale during the regional metamorphic process. Shale forms into schist as temperatures rise from about 200 degrees celsius to about 400 degrees celsius and with a pressure increase from a depth of about 8 kilometers to a depth of about 15 kilometers (as seen in metamorphic index mineral diagrams).

Associated Rock types: Garnet can be found in metamorphic rocks such as gneiss, schist and eclogite. It can also sometimes be found in igneous rocks such as granite

Occurrence in North America: Alaska, Idaho, Colorado, New York, Connecticut, Massachusetts, Maine, Nevada, and North Carolina.

Economic Uses: Its primary economic uses is as a gemstone in jewelry and other adornments. Garnet has been used as a gemstone for over 5,000 years, appearing in the archaeological record in Ancient Egyptian burial sites and was a very popular gemstone in Ancient Roman culture (1). Almandine is the most popular garnet, however, due to how abundant it is compared to other varieties of garnet. It is so abundant the cost of jewelry that uses it is relatively inexpensive. Almandine is used in all types of jewelry. It is sometimes put into a rock tumbler to make smooth beads for necklaces and bracelets, when it is rounded and polished it is called a cabochon. The gemstone comes in many different shapes including but not limited to; round, pear shaped and heart shaped. Almandine is usually cut in a medium size for jewelry and are cut shallow to allow light to pass through to highlight its unique vibrant red color.

Industrial Uses: Aside from Almandine being used as a beautiful gemstone for the last thousands of years, in the last 150 years it has garnered multiple industrial uses as well. In the United States Almandine is used for waterjet cutting, abrasive blasting, water filtration, and as an abrasive powder (1). Almandine is also used as an indicator mineral for mineral exploration and for geologic assessments.

Almandine has a hardness of 7 - 7.5 on the Moh’s Scale of Hardness. Garnet is an effective abrasive when crushed or cut and used for sanding and cutting due to its hardness. When the garnet is broken down to granule size the pieces are then bonded to paper which create a red colored sandpaper (1). This is mainly used in woodworking shops.

The largest industrial use of Almandine in the United States is waterjet cutting (1). A waterjet cutter is essentially a pressure washer. This machine pushes out a high-pressure jet stream of water. Mixed in the water is abrasive granules of the garnet. The jet stream of the mix is to be aimed at stone, ceramic, or metal (1). The jet is very powerful and at low temperature it is used for cutting into the material producing little dust (1). Waterjet Cutting is known to be used for mining and other industrial uses.

Abrasive blasting, also known as sand blasting, is done with almandine granules. Abrasive blasting uses the granules mixed in with either air or water that is then used as a propellant stream against a surface, such as metal, brick, or stone. When the mixture hits the surface it is used to either clean, smooth or remove oxidation (1).

Garnet granules can also be used as a filtration aid. When the granules are cut into particles small enough, approximately 0.3 millimeters, you fill a container that liquid flows through (1). The spaces between the particles of the garnet are big enough to let water pass through but not large enough to stop some of the pollution to pass through. This garnet is preferred because it is difficult to move and has high gravity. Due to its inertia and hardness this reduces particle abrasion during back-flushing and bed expansion (1).

First Notable Identification: Georguis Agricola (Georg Bauer) named the mineral Almandine in 1546 after an ancient gemstone cutting center in Albanda, Turkey (4). This is where Almandine supposedly had been fashioned into gemstones (4).

How We Identified It: Almandine is usually very red and hard. Its hardness its 7-7.5, so it scratches glass. Almandine crystal form is many times dodecahedron, even when found inside a rock.

Don’t Confuse It With: Ruby, Spinel, Grossular and Pyrope these are mostly confused with almandine due to color. They all can be deep shades of red. Grossular and Pyrope are types of garnet, and the ones that are most commonly confused with garnet. These two types have similar hardnesses to almandine, and all types of garnet can have the same crystal forms, which is another way that they are easily confused.