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° 04’ 51” South, 141° 11’
31” 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.