Clark and others 2007 USGS Data Series 231
Spectral Library splib06a Sample Description

(For further information on spectroscopy, see: http://speclab.cr.usgs.gov)

TITLE: Magnesite+Hydromagnesite HS47 DESCRIPT

DOCUMENTATION_FORMAT: MIXTURE

SAMPLE_ID: HS47

MIXTURE_TYPE: Intimate

MIXTURE: Magnesite + Hydromagnesite (Calcite group)

FORMULA: MgCO3 + Mg5(CO3)4(OH)2*4H2O

FORMULA_HTML: MgCO₃ + Mg₅(CO₃)₄(OH)₂•4H₂O

COLLECTION_LOCALITY: Victorville, CA

ORIGINAL_DONOR: Hunt and Salisbury Collection

CURRENT_SAMPLE_LOCATION: U.S.G.S. Denver Spectroscopy Laboratory

ULTIMATE_SAMPLE_LOCATION: U.S.G.S. Denver Spectroscopy Laboratory

SAMPLE_DESCRIPTION:

Forms series with Gaspeite and with Siderite.

Original spectrum published in:
Hunt, G.R., J.W. Salisbury, 1971, Visible and near-infrared spectra of minerals and rocks: II. Carbonates. Modern Geology, v. 2, p. 23-30.

With the following notes:
"Magnesite, MgCO₃, is a carbonate mineral commonly found in veins, and derived from the alteration of serpentine. Such magnesites are compact and not recognizably crystalline. Crystalline magnesite is found in sedimentary rocks, where it has formed either by primary precipitation or by alteration of dolomite."

"This sample is of the compact, non-crystalline variety. It appears mineralogically pure in hand specimen, but its spectrum displays bands other than those due to the carbonate ion at the longer wavelengths. These bands near 1.4 and 1.9µm are due to water of hydration and so the sample must be, at least in part, composed of hydromagnesite, roughly Mg₅(OH)₂(CO₃)₄•4H₂O. X-ray diffraction indicates only magnesite, but heating to 800°C shows the presence of 1.78% H₂O by weight, confirming the presence of some water of hydration. The water band near 1.9µm precludes observation of the three weaker vibrational bands of the carbonate radical."

END_SAMPLE_DESCRIPTION.

XRD_ANALYSIS:

pure magnesite - Hunt and Salisbury as noted in above sample description.

40 kV - 30 mA, 6.5-9.5 keV
Reference: JCPDS #8-479
Found: Member of the calcite group, probably magnesite, plus at least one additional, poorly crystallized phase.
Sought but not found: brucite

Comment: sharp pattern of crystalline carbonate closely matches the JCPDS card for magnesite. I don't understand why the (012) reflection is missing - but it doesn't appear on the JCPDS card, either. There is a moderately weak, broad peak (doublet) at 21 degrees 2 theta - thus the comment about the additional phase. Could opaline silica be present??"

J.S. Huebner, J. Pickrell, and T. Schaefer , 1994, written communication.

END_XRD_ANALYSIS.

COMPOSITIONAL_ANALYSIS_TYPE: None # XRF, EM(WDS), ICP(Trace), WChem

COMPOSITION_TRACE:

COMPOSITION_DISCUSSION:

None.

END_COMPOSITION_DISCUSSION.

MICROSCOPIC_EXAMINATION:

END_MICROSCOPIC_EXAMINATION.

SPECTROSCOPIC_DISCUSSION:

END_SPECTROSCOPIC_DISCUSSION.

SPECTRAL_PURITY: 1a2a3a4_ # 1= 0.2-3, 2= 1.5-6, 3= 6-25, 4= 20-150 microns

LIB_SPECTRA_HED:	where	Wave Range	Av_Rs_Pwr	Comment
LIB_SPECTRA:	splib04a r 2853	0.2-3.0µm	200	g.s.=
LIB_SPECTRA:	splib05a r 8596	0.2-3.0µm	200	g.s.=
LIB_SPECTRA:	splib06a r 25848			g.s.=
LIB_SPECTRA:	splib06a r 25861			g.s.=