"...The basic structural unit of the amphibole group is made up of two double chains of silica tetrahedra and a strip of octahedra. These are oriented such that the apices of the double chain tetrahedra point toward each other and sandwich the octahedral strip between them. The resulting unit is termed an “I-beam” because of its appearance in cross section . The length of the I-beam can extend indefinitely.Within the octahedral strip are three similar octahedral sites (labeled M1, M2, and M3) that may accommodate different size cations such as Fe, Mg, Al and Ti. These octahedral cations link the tetrahedral chains to form the “I-beams.” Between the I-beams is the larger, more irregular M4 site, which accommodates larger cations such as Na and Ca, as well as Fe and Mg. Cations in the M4 site cross-link the I-beams in the a and b crystallographic directions. Nestled in the center of the rings of tetrahedra formed by the double chain (Figure 5) are the O3 and A sites. In the cross-sectional view of the I-beam , the O3 site lies at the level of the apical oxygen of the tetrahedra and the A sites lie at the level of the basal oxygen of the tetrahedra. The O3 site is typically occupied by anions such as hydroxyl, fluorine, chlorine, or oxygen, and the A site by large cations such as Na or K. ..."

[CALIFORNIA GEOLOGICAL SURVEY GUIDELINES FOR GEOLOGIC INVESTIGATIONS OF NATURALLY OCCURRING ASBESTOS IN CALIFORNIA 2002]

Amphobile Regulatory	Mineral Name Amphobile Sub Group	Chemical Formula
Actinolite	Actinolite Calcic monoclinic	Ca2(Mg, Fe2+)5Si8O22(OH)2
Amosite	Cummingtonite - Grunerite Mg-Fe-Mn-Li monoclinic	Mg2Mg5Si8O22(OH)2 - Fe2Fe5Si8O22(OH)2
Anthophyllite	Anthophyllite Mg-Fe-Mn-Li orthorhombic	Mg2Mg5Si8O22(OH)2
Crocidolite	Riebeckite Sodic monoclinic	Na2(Fe2+3, Fe3+ 2 )Si8O22(OH)2
Tremolite	Tremolite Calcic monoclinic	Ca2Mg5Si8O22(OH)2
Amphobile Non-Regulatory	Mineral Name Amphobile Sub Group	Chemical Formula
Richterite	Richterite Soda - Calcic monoclinic	Na(Ca,Na)Mg5Si8O22(OH)2
Winchite	Winchite Soda - Calcic monoclinic	(Ca,Na)(Mg4Al)Si8O22(OH)2 - Fe2Fe5Si8O22(OH)2

A United States Geological Survey(USGS) study identified several amphiboles including winchite, richterite, tremolite,magnesioriebeckite, magnesio-arfvedsonite, and possibly edenite in Libby vermiculite. The most abundant of these were winchite, richterite, and tremolite. All of the amphiboles observed, with the possible exception of magnesioriebeckite, can occur in the fibrous or asbestiform habit.