ALBITITE
Albitite
is an uncommon metamorphic rock. The sample below is a slightly pyritic
albitite from Thackaringa in the Broken Hill Block of far-western New South
Wales, southeastern Australia. It looks like a quartzite, but it's
not. It consists principally of light-gray & translucent albite
(sodium-rich feldspar - NaAlSi3O8) (fine striations on
cleavage planes are clearly visible throughout the rock when viewed through a
microscope) plus some dark-gray & translucent quartz (SiO2).
Disseminated specks of brassy-colored pyrite (FeS2) are also
present. The yellowish coloration along the margin of the specimen is a
weathering rind.
Albitite at Thackaringa is only one of many
metamorphic rocks making up the Thackaringa Group (lower Willyama
Supergroup), a high-grade metamorphic succession that's late Paleoproterozoic
in age (~1.688 to 1.710 million years old). Australian workers have
identified at least two episodes of intense metamorphism of these rocks - one
at about 1.660 billion years ago, and one at about 1.599-1.600 billion years
ago.
There's disagreement about the precursor rock
(protolith) of this albitite. Before metamorphism, it was possibly a
succession of tuffs and arkoses, representing a nonmarine rift-valley fill.
Locality:
Thackaringa outcrop - natural exposures at the far-western end of Pine Ridge,
south of the Barrier Highway, near railroad tracks, ~28-31 km west-southwest of
the city of Broken Hill (= locality of O'Callaghan, 2001 - Sulfides in
Regionally Metamorphosed Terrains: the Broken Hill Block, NSW. Honors
Thesis. Australian National University, Canberra, Australia. pp.
10-12) (vicinity of 32¡ 04Õ 51Ó South, 141¡ 11Õ 31Ó East).
Albitite
(9.2 cm across) from Thackaringa, Broken Hill Block, New South Wales,
Australia. It's composed principally of albite feldspar plus some quartz
and disseminated small crystals of pyrite.
Collected & generously donated by Molly Tannian.