Amphibole

Ancient jade carvings from Chinese palaces and Maori sites in New Zealand may be the best known of amphibole’s gifts, but in our modern world another gift, asbestos, has surpassed jade’s value. Amphiboles are only one of the mineral sources for jade and asbestos though. Some jade is formed of a pyroxene mineral, while most modern asbestos is mined from chrysotile, a fibrous variety of serpentine.

Description and Identifying Characteristics

An important group of rock-forming minerals in igneous and metamorphic rocks, amphiboles share a similar crystal structure and cleavage pattern, but contain different proportions of sodium (Na), calcium (Ca), iron (Fe) and magnesium (Mg), which substitute for one another in its crystal structure. Amphibole minerals are generally dark-colored, hard, and are so similar in appearance that often they can only be distinguished from one another by examination under an optical microscope. They are important components of many intermediate igneous and metamorphic rocks, but are easily confused with pyroxene minerals that exhibit the same hardness and dark coloration. This difficulty in distinguishing amphibole from pyroxene is reflected in the group’s name. Amphibole comes from the Greek word amfibolos, which means ‘ambiguous’ or ‘doubtful’.

Ambiguity also surrounds the older name for part of the mineral group. Although it is no longer used as a mineral name, the most common rock-forming amphiboles were once called ‘Hornblende’. Hornblende was an old German mining term that comes from the German words ‘horn’ and ‘blenden’ (‘to blind’ or dazzle), references to the minerals’ hardness and glassy luster. With amphibole’s opaque character and dark color, this glassy luster is easily mistaken as being metallic, a source of confusion for modern students as well as early German miners. In fact, some sources believe that the old name more properly refered to the mineral’s deceptive appearance. It looked like a metallic ore, yet failed to yield any metal.

Amphibole minerals are typically black to dark green in color, although those found in basaltic rocks are characteristically dark brown. The dark color, hardness and well-developed cleavage patterns usually serve to distinguish these minerals from other common rock-forming minerals, with the exception of the pyroxene group. In theory, the two mineral groups can be distinguished by the angle at which their two sets of cleavage planes meet (56^o and 124^o for amphiboles, 87^o and 93^o for pyroxenes). In practice, this difference may be difficult to determine. In some igneous rocks, where the amphibole minerals formed as an alteration of pyroxenes, the identification may be even more difficult if the amphibole crystals mimic the shape of the pyroxene they replace. It is sometimes possible to distinguish amphibole crystals by their six-sided crystal cross sections, but more often the rock’s other mineral composition offers a better clue for field identification. Amphibole minerals are more common in intermediate to felsic igneous rocks, while pyroxenes are more typical of basic igneous rocks. Pyroxenes are relatively scarce in metamorphic rocks, with the exception of diopside pyroxene, which does occur in metamorphosed carbonates. In schist or gneiss rocks, however, a dark, hard mineral is more likely to be an amphibole.

Occurring in most intermediate and felsic igneous rocks, amphibole minerals often form as an alteration of pyroxene minerals during late, water-wet stages of igneous activity. As a consequence, amphibole minerals are most abundant in igneous rocks that form deep beneath the Earth’s surface than in volcanic igneous rocks. The high pressure of these subterranean settings aids the incorporation of OH-groups into the silicate crystal structure, transforming pyroxenes to amphibole. An exception to this is a variety of titanium-rich oxidized amphibole that usually occurs in basaltic rock. While amphibole minerals are common in granites, they are particularly abundant in syenite and diorite rocks, where they may comprise up to 20% of the rock volume. Amphibole minerals are also common in metamorphic schists and gneisses, and make up the bulk of the aptly named metamorphic amphibolites. In gneisses, amphiboles are the dominant minerals of the rocks’ dark bands.

As the amphibole minerals primarily form as integral parts of larger rock masses, it is unusual to find them as isolated, large, well-developed crystals. Amphibole crystals, however, do occur in pegamatites associated with diorite or syenite igneous rocks. In igneous rocks, amphibole minerals are usually associated with the intermediate feldspar minerals, while in metamorphic rocks they are associated with mica minerals. In metamorphic rocks, amphibole minerals may themselves be altered into other iron-magnesium silicates such as chlorite, epidote and biotite.

Most of the common, rock-forming amphibole minerals have relatively little economic value on their own. Their sole economic contribution is being part of the dark mineral component of the granites, diorites and syenites widely used as decorative rock and building stones. 

A relative lack of economic importance does not hold true for some less common members of the amphibole mineral group. Amphiboles include both the original jade gemstones exquisitely carved by Chinese and Maori artisans and many of the minerals collectively known as asbestos. There are actually two minerals known as jade in the gemstone trade. Nephrite (an amphibole) and jadeite (a pyroxene) are nearly indistinguishable by someone new to gems. Both exhibit the same green color and remarkable durability that has made jade a favored gemstone for jewelry and statues. Among the Aztec, jade was more valuable than gold.

As with jade, the amphiboles are not the only minerals commonly known as asbestos. These appear to have been the first minerals to be called by that name though, so they could be considered as being the original asbestos. Until recently, asbestos was primarily a novelty. A fibrous mineral that could be woven into cloth, asbestos was an exotic curiosity for the rich, notable for its fire resistant properties. The Romans knew it as linum vivum, ‘immortal linen’ and used it to wrap the bodies of distinguished citizen before cremation. As the body burned, the asbestos cloth remained untouched by flame and intact, preserving the ashes of the dead to be collected after the fire was extinguished. Later, the Emperor Charlemagne is said to have had an asbestos tablecloth that was after a meal would be tossed onto a fire to impress the Emperor’s guests. Flames would burn the food and stains away, leaving the tablecloth clean and untouched. By the late nineteenth century, asbestos had made its way down the economic ladder to common folk, although it still was something of a novelty. As recorded in the second series of ‘Little House on the Prairie’ books written by Rose Wilder, (Laura Ingalls Wilder’ daughter) asbestos mats were used by American frontier families to protect wooden surfaces from hot cookware. Asbestos cloth was also used as lamp wicks.

It was the use of asbestos for automobile break linings and gaskets that first transformed asbestos into an important industrial commodity. Its durability and heat resistance soon led to asbestos becoming one of the more widely used materials for fireproofing, heat resistance and insulation. It was used in electric stoves and hotplates, to wrap pipes, insulate buildings and create fire protective gear. Incorporated into housing shingles, wallboard and floor tiles, asbestos soon became essential to the building industry until people started to become aware of its potential health risks. However, although they were the first minerals to be known as asbestos, the amphibole minerals only account for roughly a tenth of modern asbestos production. Most commercial asbestos comes from chrysotile, a fibrous variety of the mineral serpentine.

Although most asbestos comes from chrysolite, rather than amphiboles, the widespread past use of asbestos poses some serious health risks. The same fibrous character of the asbestos minerals that allow them to be woven into fabric or used as insulation also allows small fibers to be easily broken off. If inhaled, these fibers can lodge in the lungs’ lining. Exposure to asbestos fibers can significantly increase the risk of lung cancer.

Amphibole is a common component of plutonic and metamorphic rocks across the region, but there are no notable local occurrences of collectable grade amphibole mineral crystals. The Morton Gneiss of central Minnesota is probably the most famous amphibole-bearing regional rock. The dark bands in this rock are primarily composed of amphibole and biotite, as well as dark inclusions that originally formed as volcanic fragments. These inclusions yield radiometric dates of 3.6 billion years and, for many years the Morton Gneiss was distinguished as containing the oldest known rock of a terrestrial (non-meteorite) origin. Its age has since been surpassed by other rocks found in the Canadian Arctic, but the Morton Gneiss still remains a tangible physical reminder of the earliest stages of our planet’s history. The intricate patterns of dark and red bands also make it a prized decorative building stone used across our region and marketed internationally.

Amphibole minerals’ defining characteristics are their dark color, hardness and well-developed cleavage. These usually serve to distinguish them from most other common rock-forming minerals with the exception of the pyroxene mineral group. However, some amphiboles exhibit a particularly good vitreous luster and may be mistaken as metallic minerals. The presence and orientation of cleavage planes is the best way to resolve any uncertainty.

Pyroxenes:

Chemically, the most significant differences between the pyroxene and amphibole groups are the addition of O- and OH-groups in the amphiboles, and the two groups' different silicate structures. The pyroxenes are single chain silicates, while the amphiboles are double chain silicates. In general, pyroxene crystals tend to be stubbier than the more elongated amphibole crystals, but the crystal shapes may be very similar in those amphiboles that formed from the alteration of pyroxenes. As a result, the only definitive way to distinguish the two groups is by the angle between cleavage faces on crystal fragments. On pyroxene fragments, the cleavage faces tend to meet at nearly right angles. In contrast, amphibole cleavage fragments have cleavage faces that meet at angles of nearly 60 degrees and 120 degrees. If you look down the long axis of a cleavage fragment, pyroxenes will tend to have rectangular cross-sections, while amphiboles will exhibit a diamond- or wedge-shaped pattern.

Magnetite:

Samples of dark vitreous amphibole may be mistaken as having a metallic luster. This can cause them to be confused with magnetite or other black metallic minerals. However, none of these metallic minerals will exhibit the two well-developed cleavage directions present in amphibole minerals. Magnetite is also easily distinguished from amphibole by its magnetic character.

Tourmaline:

The tourmaline group is another common accessory in many metamorphic rocks that may mimic amphibole's color and hardness. However, tourmaline minerals have a distinctive triangular cross-section and lack the amphibole minerals' well-developed cleavage.