contact metamorphism

• Contact metamorphism occurs at temperatures of between 200 to over 1000 dc.
• Argillaceous low grade rock forms spotted rock, it forms between 200-500 and is a porphyroblastic texture which contains needles up to 2cm long that show a random orientation from crystalising without direct pressure. Its groundmass is mostly mica. There are no relic structures like bedding. 
• High grade argillaceous rock forms hornfels at temperatures between 500-650 dc. Its tough, fibrous and splintery with a crystalline texture, andalusite porphyroblasts are sometimes present. 
• Limestone recrystalises to form marbles where all fossils and pre-existing structures are destroyed. Marbles show granoblastic texture where the crystals are the same size. The grain size increases with grade: low grade = <1mm; medium = 1-2mm; high >2mm. 
• Limestones like chalk are chemically simple, made of calcium carbonate in mineral calcite form. So no new minerals form, calcium carbonate recrystalises in a coarser form. 
• Orthoquartzite are chemically simple rocks mainly made of quartz. No new minerals form from pure sandstone because there are only silicon and oxygen molecules present so it recrystalises in a coarser form. Grain size increases with grade: low grade <1mm; medium = 1-2mm; high >2mm. 
• Marble and metaquartzite show triple point junctions where minerals have random orientations from recrystalising with no pressure. 3 crystals meet with 120º between angles. 
• When orthoquartzite metamorphosises it makes a rock called metaquartzite. all fossil detail and older structures are destroyed. It shows a granoblastic texture and is very resistant to weathering.