Under different conditions, a given mineral may grow as a massive aggregate where its many crystals show no distinct habit (too small or too tightly packed to develop symmetrical faces). massive magnetite octahedral magnetite
topaz amethyst SiO2 “Prismatic” and “columnar” are used inter-changeably. They describe crystals that are elongated but with an equant cross section (may be 3-, 4-, 6-sided or even more complex).
Habits of elongated crystals are described as: acicular (needle-like) prismatic (angular cross section) columnar (rounded cross section)
The elongation of crystals can be extreme... fibrous habit (fiber-like) If the fibers are flexible, the habit is called asbestiform (as with chrysotile). natrolite
Crystals may grow as tightly packed aggregates. Bundles of fibrous crystals often develop smooth, rounded surfaces. hematite • mammillary • botryoidal • reniform habits jasper malachite
Crystals that are elongated but with a distinctly flattened cross section, are said to be “bladed”. cerussite PbCO3 kyanite AlSi2O5
psilomelane Aggregates with a branching look have a “dendritic” habit. silver, Ag snow, H2O copper, Cu
Clusters of crystals can adopt many distinctive shapes. These bladed crystals of gypsum show no preferred orientation. But some clusters look strikingly like“rosettes”.
Cleavage: crystal breaking along planes Three feldspars: different colours and composition (Na-, Ca- or K-silicates) but same hardness, vitreous luster and same cleavage.
Cleavage is only visible if: • the crystal size is not microscopic • the crystal has broken up along some of these planes • Some minerals have several directions of cleavage at specific angles.
Clay minerals have sheets of strongly bonded Mg, Al, Si and O. The sheets are weakly held by van der Waals forces or hydrogen bonds. Clays should have perfect cleavage but their crystals are too small for our eye to detect it.
Micas also have structures consisting of sheets of strong bonds. Their crystals are coarser (bigger) than those of clay minerals, and can be cleaved in very thin, translucent sheets. ... micaceous cleavage!
Some mineral descriptions mention “cleavage”, others “parting”. Both are patterns of planar breaks... The distinction is in the cause of this pattern. • cleavage is present at all scales, in all specimens of a given mineral; • parting does not occur in all specimens. It depends on the growth conditions (more on that later in the course).
Hardness: it’s all relative...
1 - Talc 2 - Gypsum FINGERNAIL : 2.5 3 - Calcite PENNY : 3.5 4 - Fluorite GLASS PLATE: 4.5 5 - Apatite 6 - Orthoclase KNIFE or NAIL: 6 STREAK PLATE: 6.5 7 - Quartz 8 - Topaz STEEL FILE: 7-8 9 - Corundum 10 - Diamond Mohs hardness scale This scale is exponential.
How to test hardness on minerals... 1) Does apatite scratch quartz? Well, it left a mark on it. But... 2) The mark can be rubbed off. It was not a scratch but powder left on quartz by the softer apatite.
Check the date when you pick that penny! Pre-1997 Canadian pennies and pre-1982 U.S. pennies are nearly pure copper. Recent pennies are mostly zinc (harder than copper), thinly coated with copper to give them colour.