Pyroxene

• 1. Fire Stranger

  ›

  The name pyroxene comes from two Ancient Greek words meaning fire and stranger. Early observers found these crystals embedded in volcanic glass, sitting inside cooled lava like uninvited guests. They assumed the crystals were impurities, intruders in the glass born of fire. So they called them the fire strangers. The truth turned out to be less mysterious and more orderly. These minerals were not contaminants at all. They were early-forming crystals that had taken shape before the lava ever erupted. That small misreading hints at a larger story. How does a single family of minerals come to define so much of the rock around us? What gives pyroxenes their structure, their cleavage, and their many names? And why does the chemistry of a tiny silicon ion decide which of these minerals can exist at all? The answers begin deep inside the Earth and reach all the way to the way scientists count and classify the world.

• 2. The Silicon Chain

  ›

  Each silicon ion in a pyroxene sits at the center of four oxygen ions, forming a tetrahedron around the small silicon core. These tetrahedra link into long parallel chains. Every silicon ion shares two of its oxygen ions with its neighbors down the line. Pyroxenes are the most common single-chain silicate minerals on Earth. The only other important single-chain group, the pyroxenoids, are far less common. The tetrahedra in a chain all point the same way. That arrangement leaves two oxygen ions on one face of the chain for every single oxygen ion on the other. The narrower face carries what are called apical oxygen ions. Y cations bind pairs of chains together along their apical sides, each Y cation surrounded by six oxygen ions. Geologists have likened the resulting double chains to I-beams. These I-beams interlock, with extra X cations bonding their outer faces to neighboring I-beams and supplying the rest of the charge balance. That outer bonding is relatively weak. It is exactly this weakness that gives pyroxenes their characteristic cleavage, the clean way they split apart.

• 3. A Crowd Of Cations

  ›

  Calcium, sodium, iron and magnesium are the ions that most often fill the X site in the pyroxene formula, with zinc, manganese or lithium appearing more rarely. The Y site, by contrast, takes smaller ions. Chromium, aluminium, magnesium, cobalt, manganese, scandium, titanium, vanadium, and even iron can occupy it. Aluminium tells a revealing story about limits. In silicates such as feldspars and amphiboles, aluminium substitutes freely for silicon. In most pyroxenes that swap happens only to a limited extent. The chain structure offers great flexibility in which cations it can hold, and that flexibility is why pyroxene minerals are named chiefly by their chemistry. There is an order to filling the sites. Cations in the Y site, also called M1, bind closely to six oxygens in octahedral coordination. Cations in the X site, also called M2, can sit among six to eight oxygen atoms depending on their size. The rule for assigning ions is to work through a set sequence, placing all the silicon in the tetrahedral T site first. Leftover aluminium and then iron fill the rest of that site, with extra aluminium or iron moving to the Y site and bulkier ions to the X site.

• 4. The Quadrilateral

  ›

  Magnesium, calcium and iron pyroxenes are mapped onto a figure called the pyroxene quadrilateral. The enstatite-ferrosilite series sits here, holding the common rock-forming mineral hypersthene and containing up to 5 mol.% calcium. That series exists in three polymorphs: orthorhombic orthoenstatite and protoenstatite, and monoclinic clinoenstatite, with matching ferrosilite forms. Calcium reshapes what can crystallize. Adding more of it prevents the orthorhombic phases from forming, so pigeonite crystallizes only in the monoclinic system. There is no complete solid solution across calcium content. Pyroxenes with calcium between roughly 15 and 25 mol.% are not stable as single crystals. Instead they break apart into a pair of exsolved crystals, opening a miscibility gap between pigeonite and augite. The boundary lines themselves are partly human choices. The divide between augite and the diopside-hedenbergite solid solution is set, somewhat arbitrarily, at more than 45 mol.% calcium. Because the calcium ion cannot occupy the Y site, no pyroxene can exceed 50 mol.% calcium. The related mineral wollastonite has the formula of the hypothetical calcium end member. Its structure differs enough that it is classified instead as a pyroxenoid.

• 5. The Sodium Problem

  ›

  Sodium carries a charge of only 1+, and that single difference creates a problem the pyroxene must solve. Sliding sodium into the structure leaves a missing positive charge that has to be made up somewhere. The sodium-rich pyroxenes form a second major series, described by a pyroxene triangle rather than the quadrilateral. In jadeite and aegirine, the structure balances itself by placing a 3+ cation on the Y site. Jadeite uses aluminium for this. Aegirine uses iron in its 3+ state. Other balancing schemes exist beyond the sodium example. One couples a 1+ ion on the X site with a 3+ ion on the Y site, the route that gives jadeite its composition. Another pairs a 1+ ion on the X site with equal numbers of 2+ and 4+ ions on the Y site. A third, the Tschermak substitution, puts a 3+ ion on both a Y site and a T site. In nature, more than one of these substitutions can appear in a single mineral. Sodium pyroxenes carrying more than 20 mol.% calcium, magnesium or iron components are named omphacite and aegirine-augite. When those components reach 80% or more, the mineral is classified back on the quadrilateral diagram.

• 6. Naming The Family

  ›

  Twenty mineral names in the pyroxene group are recognized by the International Mineralogical Association's Commission on New Minerals and Mineral Names. Another 105 previously used names have been discarded. The family splits cleanly by crystal system. Pyroxenes that crystallize in the monoclinic system are the clinopyroxenes. Those that crystallize in the orthorhombic system are the orthopyroxenes. The clinopyroxenes form the longer roster. Among them are aegirine, augite, clinoenstatite, diopside, esseneite, hedenbergite, jadeite, jervisite, johannsenite, kanoite, kosmochlor, namansilite, natalyite, omphacite, petedunnite, pigeonite and spodumene. The orthopyroxenes are fewer. They include enstatite, hypersthene, ferrosilite, donpeacorite and nchwaningite. Two of their members are defined as intermediates. Bronzite sits between enstatite and hypersthene, while eulite lies between hypersthene and ferrosilite.

• 7. Inside The Earth

  ›

  Olivine and pyroxene minerals make up most of Earth's upper mantle. These are the minerals of the planet's hidden interior, the bulk of the rock beneath the crust. Closer to the surface, pyroxenes help build the rocks people can see and touch. Together with feldspar, they are the major minerals in basalt, andesite, and gabbro. These are the same volcanic and igneous rocks where the first observers once found pyroxene crystals locked in glass. Those crystals, mistaken for strangers born of fire, are in fact among the earliest minerals to take shape as molten rock begins to cool.