Cl-rich biotite and amphibole from Black Rock Forest, Cornwall, New York

Abstract

Other| April 01, 1996 Cl-rich biotite and amphibole from Black Rock Forest, Cornwall, New York Albert Leger; Albert Leger American Museum of Natural History, Department of Earth and Planetary Sciences, New York, NY, United States Search for other works by this author on: GSW Google Scholar Carolyn Rebbert; Carolyn Rebbert Search for other works by this author on: GSW Google Scholar Jim Webster Jim Webster Search for other works by this author on: GSW Google Scholar American Mineralogist (1996) 81 (3-4): 495–504. https://doi.org/10.2138/am-1996-3-423 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Albert Leger, Carolyn Rebbert, Jim Webster; Cl-rich biotite and amphibole from Black Rock Forest, Cornwall, New York. American Mineralogist 1996;; 81 (3-4): 495–504. doi: https://doi.org/10.2138/am-1996-3-423 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Biotite and amphibole containing up to 4.6 and 5.1 wt% Cl, respectively, are found in gneisses from Black Rock Forest, located within the Hudson Highlands of New York. The area was metamorphosed to granulite facies during the Grenvillian orogeny at ~ 1100 Ma. Small, subeconomic magnetite deposits coexist with the gneisses.Cl contents in biotite range from 0.1 to 4.6 wt%. Some individual biotite grains are spectacularly zoned, with Cl contents varying from 1.0 to 4.6 wt% within <20 μm. In zoned biotite, an increase in Cl content correlates with increasing Fe and decreasing Mg, Ti, and F. When data from five specimens are compared, however, Cl contents in biotite show only two strong correlations: Cl contents correlate positively with Fe (r2 = 0.75) and negatively with Ti (r2 = 0.84). Cl and Mg are uncorrelated for the complete dataset but show strong correlation within individually zoned grains (r2 = 0.74). F contents show only one correlation, a positive one with Mg (r2 = 0.62). The structural formula for the most Cl-rich biotite analyzed is (K2.00Na0.02)(Mg1.94Fe3.90Ti0.07Al0.04)(Si5.85Al2.15)O20(OH2.46F0.25Cl1.29), as normalized to 24 (O + OH + F + Cl). This biotite is one of the most Cl rich ever reported.Cl contents in amphibole range from 0.2 to 5.1 wt%. Cl contents correlate positively with Fe2+/(Fe2+ + Mg) (r2 = 0.84) and K (r2 = 0.70) and negatively with Si/(Si + [4]Al) (r2 = 0.49) and Ti (r2 = 0.76). F contents are uncorrelated with any of the other elements analyzed. The structural formula for the most Cl-rich amphibole, a hastingsite, is (K0.57Na0.31)(Ca1.90Na0.10)(Fe2.692+Mg0.90Mn0.03Ti0.06Fe1.023+Al0.29)(Si5.80Al2.20)O22(OH0.55F0.07Cl1.38) as normalized to 13 octahedral and tetrahedral cations.In rocks with coexisting amphibole and biotite, Cl partitions preferentially into the amphibole and F partitions preferentially into the biotite. Infiltration of the gneisses by fluids rich in Fe and Cl (perhaps as an FeCl2 compound) could explain the formation of magnetite bodies as well as the Fe- and Cl-enriched phases found in and around Black Rock Forest. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.