Marble: the stony truth...

["Dr. Tim Palmer" <tjp@zzzzzzzzzz>]

Could I add something to the discussion about fabrics in marble?

As is generally known, the term 'marble' in sculpture and architecture
covers a wide variety of materials that are polishable.  The remarks below
refer to calcite marbles (made predominantly of calcium carbonate).  They
include hard limestones, i.e. limestones in which nearly all of the pore
spaces have been filled by precipitation of further calcite crystals after
burial of the rock (the process of diagenesis); they also include true
metamorphic marbles which are limestones that have been recrystallised
(with increasing crystal size) by local heating and pressure whilst buried
in the earth.  Most importantly, many marbles are the results of both
processes (to varying extents), and the processes themselves grade into one
another in the geological environment.  In addition, metamorphic marbles
may have formed under pressure conditions that were varyingly directional
during burial, but a strong directional element is not necessary for the
marble to form.  In the early experiments in making marble in heated sealed
gun barrels, for example, there was no directional stress.

In hard limestones and many true marbles, the individual crystals are
fairly equidimensional (more or less the same size in all
directions).  Their shapes and sizes are controlled by the space in which
they have to grow, before they bump up against their growing
neighbours.  If there are directional stresses in the overall environment,
then this can affect the orientation of the internal structure of the
charged atoms in the crystals, and there is a tendency for the crystals to
be aligned in the same way.  Since they tend to grow faster along one
particular axis, this can lead to a preferred fabric in the overall
rock.  But it is confusing to say that they only grow in one direction
(which would make then infinitely long and of negligible width and breadth).

Pure limestones give rise to pure marbles (e.g the top quality Carrara
statuary marble) that are uniformly white.  Inpurities in the original
limestones (from minute to large amounts; of organic matter, clays, iron
minerals etc), effectively get pushed aside and altered into other
low-grade metamorphic minerals in the process of marble formation
(marmorisation).  This doesn't happen to the same extent in hard limestones
that have not been influenced by metamorphic processes, in which the colour
that arises from these impurities remains uniformly distributed throughout
the rock.  But the dark streaks in most true metamorphic marbles are
concentrations of these impurities.  Many of the greyish, bluish, and
greenish swirls are rich in clay minerals, of which the individual minute
crystals are each shaped like tiny flat playing cards, orientated parallel
to the line of the swirl or vein.  They thus tend to break along these
streaks more readily than across them.  This is one source of directional
weakness in some marble.

Tim