Requiem for a Facies

It seems ironic that the hornfels facies
are being dropped as insignificant to
the facies classification, when it was
their discovery that led to the creation
of the mineral facies classification of
rocks. (However, perhaps one can now add
'hornfels' to the base terms 'schist,
gneiss, & granofels'.)

Eskola had placed little value upon
facies names, which he offered as
'provisional'. He was uncomfortable with
coining 'hornfels facies', as it had
just been used in a different sense by
Goldschmidt near Oslo. Eliminating the
name might not have bothered him, though
eliminating the facies probably would.
It eliminates a bit of history.

These hornfelses proved the existence of
mineral facies. After blending a bit of
Gibbs, Bowen, CIPW, & Grubenmann into an
hypothesis, both Goldschmidt's triangle
from Christiania and Eskola's triangle
from Orijarvi were finally decomposed
into 'tie triangles' of equilibrium
mineral assemblages. When the chemical
plots of hornfelses from the literature
faithfully fell within their minerals'
fields, the mineral facies was invented.
Each complex of tie triangles,
representing a facies, labeled a
different (equilibrium) intensive state.

To illustrate their value, Eskola just
picked some rocks whose environments
were known: a sample bouquet to
illustrate the classification. Two
contact hornfelses of differing depth he
already had; he picked one each from
Grubenmann's shallow, medium, and deep
zones; sanidine-bearing xenoliths had
been very hot & shallow; and eclogite
underlay even granulites.

>From Grubenmann came most facies names:
eclogite & granulite from his deep zone,
amphibolite from his medium zone,
glaucophane schist from his shallow
zone. Where greenschist came from, I
don't know; but Eskola defined the
facies using a green schist from
Grubenmann's shallow zone. Gabbroic
rocks (metabasites) were preferred
because Osann's ACF triangles was ideal
for them, Bowen's discontinuous reaction
series showed their solutions limited,
stability ordered; and - they created
more little tie trianges on his
diagrams.

It was Eskola's hope that people would
create their own facies names, used
locally. Facies, as opposed to most
isograds, are environmentally sensitive
but independent of composition. They are
identified in the field, using a lens,
from an association of rocks containing
typical assemblages, or the occasional
single rock containing a critical
assemblage. Facies were for everyone.

The hornfels facies will be sadly
missed.


Bruce Bathurst