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.

 


 

 

Home page