BERTHIERINE FROM THE LOWER CRETACEOUS CLEARWATER FORMATION, ALBERTA, CANADA

Berthierine occurs as pore-linings of well crystallized laths of varia ble thickness in oil-sands of the Clearwater Formation, Alberta, Canad a. Berthierine crystallized early in diagenesis within portions of a d eltaic/estuarine complex dominated by brackish to fresh water. Separat es prepared using high gradient magnetic separation contain approximat ely equal amounts of monoclinic and orthohexagonal berthierine. Minor, but variable, quantities of inseparable, iron-rich impurities mainly consist of chamosite Ib and IIb, and Fe-rich smectitic clays. Clearwat er Formation berthierine has a range of chemical compositions that dif fer from those reported for most other berthierines. The SiO2 (27-35 w t%), Fe2O3 (5-8 wt%) and Al2O3 (16-18 wt%) contents for Clearwater For mation berthierine fall between values normally reported for berthieri ne and odinite. The average structural formula of five samples studied in detail is (Fe1.012+Al0.82Mg0.46Fe0.283+ Mn(<0.001)square(0.43))(Si 1.74Al0.26)O-5(OH)(4), where square represents vacancies in the octahe dral sheet. The large number of vacancies in the octahedral sheet impl ies a di-trioctahedral character for this clay. Our results also sugge st that a series of compositions can occur between ideal berthierine a nd odinite end-members. Berthierine has been preserved within the Clea rwater Formation because temperatures during diagenesis did not exceed 70 degrees C, and perhaps also because hydrocarbon emplacement limite d subsequent transformation of berthierine to other phases, such as ch amosite. Intense, early diagenetic, microbial activity and/or the stro ngly reducing environment created by later emplacement of hydrocarbons may be responsible for the Fe2+/Fe3+ ratio of the berthierine. Becaus e of these conditions, this ratio may have changed since initial clay crystallization. The Clearwater Formation occurrence of grain-coating Fe-rich clays provides valuable insights into possible relationships b etween the Fe-serpentine minerals, odinite and berthierine, and suppor ts an important role for these phases as precursors to the grain-coati ng and pore-lining Fe-chlorite (chamosite) that is so common in ancien t sandstones, including many hydrocarbon reservoirs.