Calcite. Tonfanau.

Calcite. Tonfanau.

Calcite, Calcium carbonate, CaCO3  The club had a field trip to the coastal exposures at the small village of Tonfanau, about two miles north of Towyn, We noted a small quarry as we were leaving the village, and decided to investigate. There were several specimens of calcite crystals on the quarry floor, and this is one of them. I later discovered that the crystals were from the walls of calcite pockets in the quarry face. Although the crystals are not perfect, and maybe not very interesting, I was tempted to bring a specimen home.    

Vermiculite. Ontario.

Vermiculite. Ontario.

Vermiculite is a hydrous phylosilicate mineral, (Mg,Fe2+, Fe3+ )3(Al,Si)4O10 (OH)2 . 4H2O  This clay mineral has the property of  expansion to 8 to 20 times of its original size when heated, which makes it very useful in cement, and insulation products. It is also widely used as a soil additive in gardening products. This particular specimen is from Ontario, Canada.  

Schorl tourmaline, Norway.

Schorl tourmaline, Norway.

The general formula for tourmaline, which is a group mineral is NaFe2+3Al6(BO3)3Si6O18(OH)4  It is a Boron silicon mineral, and the black variety is called schorl, which is the sodium iron end member of the group. This Norwegian specimen clearly shows at least three elongated prismatic crystals.

Gypsum, Desert rose var. Morocco.

Gypsum, Desert rose var. Morocco.

Gypsum, CaSO4. 2H2O  Calcium Sulfate dihydrate is an evaporite mineral which is found in arid areas where salt rich lakes have dried up, leaving the flower like constructions  known as desert roses. The reddish colour is the result of sand grains incorporated into the crystals during growth. This specimen is described as coming from Morocco.

Stilbite. Isle of Skye.

Stilbite. Isle of Skye.

Stilbite is a tectosilicate mineral of the zeolite group, and is a series name rather than a specific mineral. There is a series between Stilbite-Ca and Stilbite-Na. Stilbite-Ca is NaCa4(Si27AL9)O72 . 28(H2O), and Stilbite-Na is Na9(Si27Al9)O72 . 28(H2O) Zeolites have a porous structure which can accomodate a wide range of cations, i.e. Na+, Ca2+, K+, etc.,Which explains why the zeolites are so complex, and not always easy to identify outside the laboratory. This specimen was brought back from the Isle of Skye by one of our members.