amphibole
/ AM-fih-bole /
Amphibole is the double-chain silicate, built by welding two pyroxene-style single chains together side by side into a wide ribbon. Where a single chain is a line of tetrahedra, a double chain is a ladder: two lines cross-linked at alternate rungs. Amphiboles are dark, prismatic rock-forming minerals, hornblende the most common of them, and they always carry hydroxyl (OH) groups woven into the ribbon, a detail that will matter when the mineral is heated.
Cross-linking makes half the tetrahedra share two corners and half share three, so the repeating unit averages to Si4O11 and the Si:O ratio is 4:11, between the 1:3 of single chains and the 2:5 of sheets. A representative formula is that of tremolite, Ca2Mg5Si8O22(OH)2, showing the double chain plus the balancing cations plus the built-in OH. Because the strong bonding still runs along the ribbons and the cations only tie ribbons together sideways, amphiboles cleave along two planes meeting at about 56 and 124 degrees, roughly 60/120, the classic contrast with pyroxene's near-90-degree cleavage.
Amphiboles matter to a ceramist in two honest ways. First, their built-in OH means they release water when fired and can break down, so they are minor and often unwelcome accessory minerals in raw clays. Second, and importantly, several amphiboles (crocidolite, amosite) are asbestos minerals: their double chains grow as strong, thin, breathable fibres, prized historically for heat resistance but now known to be a serious respiratory hazard. The double chain is thus both a beautiful step on the polymerisation ladder and a cautionary tale.
Tremolite, Ca2Mg5Si8O22(OH)2, is a double chain: two single chains fused, with calcium and magnesium tying the ribbons together and hydroxyl tucked inside. Heat it hard enough and the OH leaves as steam, and the mineral decomposes.
Double chain (4:11) with built-in OH: the amphibole ribbon, one rung above pyroxene on the polymerisation ladder.
Not every amphibole is asbestos, but the fibrous varieties are genuinely dangerous. Their toughness as fibres, once an asset, is exactly why the sharp needles lodge in the lungs.