Introduction to Amphibole Group Minerals
The amphibole supergroup is a group of more then hundred rock-forming silicates. This article provides general introduction to structure, physical properties and chemical composition of amphiboles.
What are amphiboles?
All amphibole mineral species have perfect cleavage in two directions and a splintery fracture. Their color typically is dark green, brown or black, although many colors, including colorless, white, yellow, green, blue, and lilac are known. Amphiboles occur in both metamorphic and igneous rocks, frequently as dark elongated grains and crystals embedded in the rock, but can occasionally form well developed crystals. Such crystals are most commonly found in pegmatites and as porphyroblasts in igneous rocks, marbles/skarns and veins within metamorphic rocks.
The dark color, crystal form, hardness, and well-developed cleavage usually serve to distinguish these minerals from other common rock-forming minerals. The pyroxene group can appear in similar environments and has physical characteristics similar to the amphiboles: the two mineral groups can be distinguished based on the angles between their cleavage planes; the amphiboles have cleavage planes with 56˚ and 124˚ and the pyroxenes have 87˚ and 93˚. Amphiboles can also be found as pseudomorphs after pyroxenes, often making the distinction even harder. Amphibole crystals can also be identified by their six-sided crystal cross sections.
The amphibole group minerals are generally considered amongst the most complex silicate groups. There are several reasons for this, but the basis for the complexity is the large chemical variation within the same molecular structure, resulting in a wide variety of mineral species with similar physical properties. The chemical variation is expressed by Hawthorne and Oberti (2006):
The chemical composition and variability of the amphiboles may be expressed by the general formula AB2C5T8O22W2, where:
- A = □, Na, K, Ca, Pb2+
- B = Li, Na, Mg, Fe2+, Mn2+, Ca
- C = Li, Mg, Fe2+, Mn2+, Zn, Co, Ni, Al, Fe3+, Cr3+, Mn3+, V3+, Ti4+, Zr
- T = Si, Al, Ti4+
- W = (OH), F, Cl, O
There are more than 100 minerals approved by the IMA as a result of this chemical variability. There are probably several tens of additional species identified, but yet to be properly described. The chemical diversity also shows within a single grain or crystal. Zoning is common within a single crystal and a locality may well contain 5 or more species. Mont St-Hilaire in Canada, is extreme in this respect as up to 10 different amphibole species with arfvedsonite root names may be present, some of which are not (yet) properly described.
The acknowledged amphibole nomenclature use defined rules of prefixes to distinguish between closely related species, giving names like potassic-magnesio-arfvedsonite and ferro-ferri-taramite. Such naming conventions are very useful in showing how closely related species relate, but can also be confusing. For example, sometimes the mineral name, such as arfvedsonite is used synonymously with the root-name arfvedsonite so it can be hard to know whether the author refers to a specific mineral or the more general root-name.
The nomenclature also allows a category of so-called named amphiboles. These are amphiboles that have been identified via Electron Micro Probe Analyzer (EMPA) analysis, but are not properly described and thus not approved by the CNMMN (Committee of New Minerals and Mineral Names, which is one of the committees under the International Mineral Association - IMA).
The principles for assigning a name to a named amphibole are the same as for approved minerals. The amphibole ferro-chloro-pargasite, has been identified from Lukkulaisvaara in Karelia, Russia and published as a named amphibole. It is not possible to see whether this is an approved amphibole mineral or simply a named amphibole based on the name alone. One has to look up in the IMA list of approved minerals to make sure.
Because of the complex chemistry (and the resulting complex nomenclature), amphiboles require careful and sophisticated analytical work to be properly characterized and identified. The most common and inexpensive techniques for analysis amongst collectors, EDS (Energy Dispersive X-ray Spectroscopy), WDS (Wave-length Dispersive X-ray Spectroscopy) and XRD (X-ray Powder Diffraction) are only suitable for approximate identification of most amphibole compositions, although they can be helpful for distinguishing some of the near-end member compositions. EDS/WDS as well as XRD will easily distinguish between anthophyllite and tremolite from a mafic rock, and it may give approximate identifications indicating that a certain amphibole is hastingsitic or richteritic, these techniques cannot be used to separate magnesio-hastingsite from ferro-hornblende to state an example.
Even the most commonly used method amongst scientists, the Electron Micro Probe Analyzer (EMPA) can provide inaccurate results. EMPA does not analyse for H or Li, which both can be important for identification of the species. Other important elements such as F, Cl, Mn and others are often omitted from the analysis, giving slightly distorted normalized formulas that can lead to mis-identification. This creates uncertainty in the identification of minerals near the borderline of a solid solution series, or in other words most naturally occurring amphiboles. However, the greater problem is that EMPA does not distinguish between Fe2+ and Fe3+, which is very important for identification of most amphibole species. This ratio is normally calculated, and good spreadsheets are available (Locock 2014), but the results are inaccurate and may lead to inaccurate identification. Accurate identification of an amphibole requires a combination of methods, such as XRD, EMPA, LA-ICP-MS and Mössbauer spectroscopy, but the complexity of absolute identification means that this is rarely done. Atlases of XRD, IR and Raman spectra for amphiboles does not get any better than the original identification of the samples used for such tables, and are consequently of limited use.
Although the group is certainly complex, there is no reason to give up on it. As collectors, we need to spend some time studying the group, and we must accept the fact that it will not always be practical or even possible to identify the correct specie(s) contained in a specimen. Personally, I label amphiboles with what I consider reasonable certainty. For tremolite and other common amphiboles, and amphiboles that are dominant at their locality, I can label a specimen with a species name. More exotic amphiboles or amphiboles from poorly described localities or with chemistry that is hard to analyze I often label a specimen amphibole or use adjectival modifiers as recommended by the amphibole sub-committee of the CNMMN. I find nothing wrong with the terms kearsutitic amphibole or barrositic amphibole in cases where I have specific amphiboles from a known locality but lack sufficient analytic evidence.
Any exotic amphiboles offered for sale should be accompanied with proper analytical data or at least literature reference for the locality allowing the buyer/collector to judge the likelihood of an identification. Otherwise any identification should be treated with high caution.
The Amphibole Atomic Structure: The Double Chain
The complexity and flexibility of the amphibole molecule is the root cause for the complexity of the amphiboles. The basic building block for the amphiboles is the same SiO4 tetrahedra that are the building block and core of all silicates. In the amphiboles, these tetrahedra form long double chains:
Between and associated with these chains, other elements find their space. Figure 2 shows an approximate representation of the positions of these other elements, which in this case constitute the generic formula of amphibole: AB2C5T8O22W2.
In both figure 2 and 3, the circles represent the position and an approximate size for typical ions filling the voids between the double chains as represented in the general formula of amphibole.
- The orange sphere represents the A position, shown with a size roughly similar to the relative size of Na.
- The green spheres represents the B position, (also called M4), shown with a size roughly similar to the relative size of Ca.
- The different shades of blue represent the C position (also called M1 to M3.), are shown with a size roughly similar to the relative size of Mg.
- The grey triangles represent the SiO4 tetrahedra, the T8O22 part of the formula. Often, some Si is replaced by Al.
- Red spheres represent the W position, which is normally mostly (OH), but can also be F, Cl and O. Their size is roughly similar to the relative size of (OH).
It is the flexibility of the double chains that results in the chemical complexity of the amphiboles. The chains can be distorted, thus allowing ions of different size and charge to fill the different voids. Figure 4 illustrates some of this flexibility by an example given by Heaveysege et al. (2015). Compared to the ideal double chain presented in figure 1, this is clearly distorted, allowing different elements to fill different positions.
The Example of Tremolite and Arfvedsonite
Tremolite, □Ca2Mg5Si8O22(OH)2 and NaNa2Fe2+4Fe3+Si8O22(OH)2 are both amongst the more common amphiboles. Looking at the chemical formula, the similarity with the general amphibole AB2C5T8O22W2 can easily be seen, yet the chemistry of the two amphibole species are quite different: