Browse Items (6 total)

Formula FeS2 Structure Non-silicate Crystal System Crystal system: cubic Physical Properties Colour in hand specimen: pale brassy yellow; black when fine-grainedStreak: greenish or brownishLustre: metallicCleavage: not…

Formula Na[AlSi3O8] – Ca[Al2Si2O8] Structure Framework silicate Crystal System Crystal system: triclinic (+) (-) Physical Properties Colour in hand specimen: white Cleavage: two at 90° Hardness: 6-6.5 Optical Properties Colour in…

Formula (K,Na)[AlSi3O8] Structure Framework silicate Crystal System Crystal system: monoclinic (-) or triclinic (+) (-) Physical Properties Colour in hand specimen: white or pink Cleavage: two at 90° Hardness: 6-6.5 Optical…

Hand Specimen Mostly moderately coarse-grained (typically 1-2 mm), some regions finer-grained (typically ~0.5 mm). “Salt and pepper” appearance being speckled black and white. Two black phases, biotite mica and hornblende amphibole. White…

NB, the term ‘obsidian’ refers to a jet-black glassy rock. It usually has a rhyolitic composition, but not always. Hand Specimen Black rock with the characteristic vitreous lustre and conchoidal fracture of glass. The surfaces are…

Hand Specimen This rock has a porphyritic texture. Phenocrysts of white feldspar up to 30mm long are surrounded by a coarse groundmass of quartz, biotite and white feldspar. Thin-section Alkali feldspar (orthoclase) - Grains typically 2-4mm…