#acicular

What makes bismuthinite special:

• Silvery-gray to tin-white metallic luster that tarnishes to iridescent bronze over time

• Perfect cleavage in one direction — splits into fine blades

• Softer than most sulfides (Mohs 2) — can scratch with a fingernail

• The densest common sulfide (bismuth is a heavy metal atom)

• Produces bismuth metal — used in pharmaceuticals (Pepto-Bismol), solders, and the famous iridescent hopper crystals grown by collectors

Two generations of Brochantite, one atop the other

Many of us have grown similar crystals by hanging a piece of string into a copper sulphate solution and letting it evaporate. In nature, an arid climate and an oxidising (see http://bit.ly/1I4XWKt for an explanation) copper sulphide deposit are required for this mineral, which forms when groundwater interacts with reduced ore deposits changing the primary minerals to secondary ones. Preservation is also an issue since like Atacamite, another arid mineral (see http://bit.ly/2iEOVFX), it is soluble in water and so is only rarely found.

Colour ranges from bright emerald green to a dark blue green, though its softness (3.5 on Mohs scale) prevents faceting or jewellery use. Since it forms in the same environments as malachite and azurite (copper carbonates), they are frequently found together, and sometime brochantite replaces the other minerals, taking on their crystal shape in a process called pseudomorphism (from pretend shape in Greek). The more usual distinguishing shape is as sprays of acicular (needle shaped) crystals, often radiating from a common centre. It was named in 1824 after a French mineralogist, Brochant de Villiers, and is an important ore mineral in several desert mines.

Main localities include the copper mines of the Atacama desert in Chile, the Zaire copper belt in the Democratic Republic of Congo, the Tsumeb mine (see http://bit.ly/1c2CjkD) and Arizona in the USA. A few stunning specimens (including the one in the photo that resembles an eerie mineral trilobite that once prowled over some long vanished Cambrian sea) have emerged this past decade from the Milpillas mine in Sonora, Mexico, also famous for beautiful azurites. Here, in this 2.4 x 1.8 x 1.8 cm thumbnail several deep green crystals rise above sprays of needle shaped (called acicular) light green material making for a stunning ensemble.

Loz

So fine, so straight, so sharp, so needle-like and so thickly clustered were the teeth in the decapitated head of this fish that they almost formed a fine fuzz!!! The pain felt by prey when those jaws snapped shut with full force can only be imagined!!!

This head - the beast almost snarling in death - came from the discards of fishing nets. Whether the fisherman had lopped off the more edible lower body of this fish I do not know.