Ephesite
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| Ephesite | |
|---|---|
 | |
| General | |
| Category | Phyllosilicate minerals |
| Group | Mica group, trioctahedral mica group |
| Formula | NaLiAl2(Al2Si2)O10(OH)2 |
| IMA symbol | Eph[1] |
| Strunz classification | 9.EC.20 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | C2/c[2] also reported is a triclinic polytype[3] |
| Unit cell | a = 5.12, b = 8.853 c = 19.303 [Å]; beta = 95.08°; Z = 4 |
| Identification | |
| Color | Brownish pink, pearl gray, pale green |
| Crystal habit | Flakes |
| Twinning | Commonly twinned about [310] or [310] |
| Cleavage | Perfect on {001} |
| Tenacity | Brittle |
| Mohs scale hardness | 3.5–4 |
| Luster | Vitreous, pearly on cleavage |
| Diaphaneity | Translucent |
| Specific gravity | 2.984 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.592 - 1.595 nβ = 1.624 - 1.625 nγ = 1.625 - 1.627 |
| Birefringence | δ = 0.033 |
| 2V angle | 18° to 28° |
| Dispersion | r > v strong |
| References | [2][3][4] |
Ephesite is a rare member of the mica silicate mineral group, phyllosilicate. It is restricted to quartz-free, alumina rich mineral assemblages and has been found in South African deposits in the Postmasburg district as well as Ephesus, Turkey.[5]
Ephesite has an ideal chemical formula of NaLiAl2(Al2Si2)O10(OH)2.[6] Ephesite and paragonite are closely related due to their substitution of sodium in place of potassium. The general form of most micas, which can vary such as in the place of ephesite, can be written as W(X,Y)2-3Z4O10(OH,F)2 as observed by many sources. In the case of ephesite the W compound is sodium and the (X,Y) is lithium and aluminium, it also bears two hydroxides as the end members.[5]
Structure
Ephesite, a mica structure, arises from the stacking of T-O-T layers along the c-axis direction connected by I-cations where T,O, and I stand for tetrahedral-, octahedral-, and interlayer. Creating long, flat sheets of sodium and lithium rich tetrahedra.[7] Ephesite in particular belongs in the trioctahedral mica subgroup. Micas are determined and named for their end members and species that define a wide range of compositions. Depending on the interlayer cation, the micas are subdivided into true micas (if 50% I cations present are monovalent) or brittle micas ( if > 50% I cations present are divalent). Ephesite with monovalent cations of Na prove to be a true mica and with 2.5 octahedral cations are trioctahedral. Ephesite is classified as an uncommon true non-K mica of which only 2.1% of the micas are categorized.[6]
