[...]... Prisms have a square cross section (a basal section i square) *cLEAVAGE { 110} good appearing as traces parallel to the prism edge in prismatic sections but intersecting at right angles in a basal section RELIEF Moderate ALTERATION Andalusite can invert or change to sillimanite with increasing metamorphic grade Under hydrothermal conditions or retrograde metamorphism andalusite changes to sericite (a type... particularly in quartz carbonate veins and in alte red lava flows Clay minerals Phyllosilicates The clay minerals are extremely important in weathering processes Many primary igneous minerals produce clay minerals as a final wea thering product Feldspars particularly give rise to clay minerals ; plagiocl ase feldspar reacts with water to give montmorillonite, and orthoclase feldspar in a similar way produces... is almost black Absorbing phases (opaque) , e.g galena (C), appear white Holes, pits and cracks appear black Note the black triangular cleavage pits in the galena and the abundant pits in the barite which resu lts, not from poor polishing, but from the abund ant fluid inclusions Scratches appear as long straight or curving lines They are quite abundant in the galena which is soft and scratches easily... whereas iron-rich and alkali amphiboles usually are strongly coloured and pleochroic HABIT Amphiboles usually occur as elongate prismatic minerals, often with diamond shaped cross sections *cLEAVAGE All amphiboles have two prismatic cleavages which intersect at 56° (acute angle) RELIEF Moderate to high ALTERATION Common in all amphiboles; usually to chlorite or talc in the presence of water A typical... 100 may be omitted as this is the standard load As VHN values are always given in kglmm 2 this is also often omitted The determination of hardness is a relatively imprecise technique, so an average of several indentations should be used Tables of VHN usually give a range in value for a mineral, taking into account variations du~ to compositi~n.al variations, anistropy of hardness and uncertainty... chlorite may occur (the sign changes from +veto ve) Ve ry weak, usually with anomalous deep Berlin blue colour Bi ax ial Bxa figure on basal section with small 2V Usually positive but so me va rieties- chamosite in particular- are optically negative Interfe re nce figures are rarely obtained Stra ight to cleavage but can be oblique with small angle y or a ' cl (fast or slow to cleavage) ; very small angle(... mineral has then to be rotated to bring the cleavages parallel to the crosswire will vary from nearly oo to 9° depending on the biotite composition, and this angle is called the extinction angle The maximum extinction angle of many biaxial minerals is an important optical property and has to be precisely determined This is done as follows A mineral grain is rotated into extinction, and the angular position... vibrating polarised light To check that the polars are exactly crossed examine an isotropic substance such as glass and adjust the polariser to give maximum darkness (complete extinction) The alignment of polariser and anlyser for reflected light can be set approximately fairly easily Begin by obtaining a level section of a bright i otropic mineral such as pyrite Rotate the analyser and polariser to. .. resin cage rock slice 50 500 IJ.m thick polished surfaces 3cm 28 Grinding the surface fiat using carborundum grit and water on a glass or a metal surface Polishing the surface using diamond grit and an oily lubricant on a relatively hard 'paper' lap Buffing the surface using gamma alumina powder and water as lubricant on a relatively soft ' cloth' lap 29 2 CRYSTAL CHEMISTRY Silicate minerals 2.1 Crystal... cracks and specks of dust appear black Reflection from crystal faces in holes may give peculiar effects such as very bright patches of light Scratches on the polished surface of minerals appear as long straight or curving lines, often terminating at grain boundaries or pits Severe fine scratching can cause a change in the appearance of minerals Scratches on native metals , for example, tend to scatter . bar greater than less than greater than or equal to less than or equal to app roximately approximately equal to perpendicular to parallel. appearance to a Becke line. A sharp grain contact between. a hard mineral such as pyrite and a soft mineral such as chalcopyrite should appear as