Brucite



Brucite is a magnesium hydroxide mineral and the Mg-analogue of pyrochroite. Local material is the most manganoan known, with up to 17 wt. % MnO and 3 wt. % ZnO in solid solution for MgO in one analyzed Franklin specimen. The limits of solid solution between local brucite and pyrochroite remain uninvestigated.
Brucite was first reported from Franklin by Bauer and Berman (1930); they described it using a now defunct varietal name, manganbrucite, which was also utilized by Palache (1935). It has not been studied since. Brucite has also been found at Sterling Hill. The name brucite also was used for chondrodite prior to 1824; a history of this matter is given by Frondel (1972).
Franklin brucite occurs as acicular to stout prismatic 1-2 mm crystals with a tapering trigonal pyramidal habit and triangular cross-section. Some crystals terminate in points, but most have a pearly-lustered, slightly curved termination formed of slightly offset pinacoidal faces. Such crystals form parallel and subparallel aggregates. Bauer and Berman (1930) reported the color as varying from white to deep brown. Brucite seen by [Dunn] is of very high quality, semitransparent, and varies from colorless to very light pink and light violet, not unlike the color of very weakly colored barysilite. Highly manganoan material weathers to a black color. Some Sterling Hill brucite is intimately mixed with zincite, giving it a false orange color.
Cleavage is perfect and easily produced; crystals are easily broken. The luster of cleavage surfaces is distinctly pearly. Optically; the indices of refraction are quite variable, suggesting much solid solution, perhaps toward pyrochroite. There is no discernible fluorescence in ultraviolet. It is best identified using X-ray methods.
Brucite was first reported from Franklin in a narrow veinlet cutting franklinite-willemite ore. At Sterling Hill, brucite has been found as 1.0 cm platy crystals, intimately mixed with fine-grained orange zincite and associated with calcite, dypingite, and sussexite in the north orebody. This material weakly resembles kraisslite or a weathered mica. Brucite was reported with oxidized sulfides at Sterling Hill by Jenkins and Misiur (1994). There is a sparseness of reported occurrences, but brucite is likely more common than is recognized, due to its colorless nature and small grain size. (Dunn, 1995)


 Location Found: Franklin and Ogdensburg
     
 
 Year Discovered: 1824
     
 Formula: Mg(OH)2
 Essential Elements: Hydrogen, Magnesium, Oxygen
 All Elements in Formula: Hydrogen, Magnesium, Oxygen
     
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
     
     
 To find out more about this mineral at minDat's website, follow this link   Brucite

     
 References:
Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.585

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.46


The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 36, No. 1 - Spring 1995, pg. 9Closest-Packing and Hydrogen Bonds in Minerals of the Franklin Marble, Paul B. Moore - Brucite
View IssueV. 35, No. 2 - Fall 1994, pg. 20A Complex Base-Metal Assemblage From the Sterling Mine New Jersey - Brucite
View IssueV. 7, No. 2 - August 1966, pg. 6The Minerals of Sterling Hill 1962-65 by Frank Z. Edwards - Brucite variety Manganbrucite (small article)
     
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