Habit refers to the general shape of mineral particles, as in the “platy” habit of clay particles, inclusive of shapes that reflect the internal arrangement of atoms as well as crystal growth features. Examples of habits include acicular, lamellar, and equant.
The time, t1/2, required for the number of radioactive atoms in a system to decay and thus decrease to half of the original number.
A poorly defined material, possibly paragonite and lithian muscovite.
An obsolete name for altered material, probably vermiculite.
A member of the kaolin group, with a chemical composition of ideally Al2Si2O5(OH)4 . x(H2O). Varying amounts (x) of H2O may be present in the interlayer, and the terms halloysite (7 Å) and halloysite (10 Å) were recommended for general usage to quantify the amount of H2O present between layers. Values of x ~ 0 [halloysite (7 Å)] are near kaolinite and x ~2 is halloysite (10 Å). Gentle heating of the halloysite (10 Å) phase will produce halloysite (7 Å), and this is a non-reversible reaction. Halloysite (10 Å) requires storage in water to prevent (partial) dehydration. Halloysite commonly has considerable stacking disorder although a “well crystallized” sample may have an approximate two-layer (2M1) structure for halloysite (7 Å) for a limited stacking sequence of 6-7 layers. Atomic coordinates for interlayer H2O are not known, although H2O within the silicate ring and H2O in a discontinuous plane between the layers have been suggested. Halloysite layers may be planar, curved, rolled (tubular), and partly spherical to spherical, and these morphologies appear to be related to crystallization conditions and chemical composition. There is no way to conclusively differentiate between halloysite and kaolinite without knowing the history of the sample, although suggestions have included the evidence of the 2M1 polytype and various treatments involving intercalation as ways to identify halloysite (7 Å).
Cf., dickite, kaolin, kaolinite, nacrite
The Hamaker constant, A, describes the attractive force between two solid surfaces due to temporary dipole forces (van der Waals interactions). For ideal planar surfaces separated by length L, the van der Waals potential energy is proportional to A/L2.
A process involving the formation of cements (e.g., Portland cement, geopolymers) during which intermediate phases [e.g., calcium silicate (CS) phases in Portland cement] (re)crystallize to new phases by hydration (e.g., Portland cement) or dehydration (e.g., geopolymer cement). Ingredients (e.g., naphthalene sulfonate, a plastifier) may be added to delay hardening.
Cf., calcium silicate (CS) phases, calcium silicate hydrate (CSH) phases, geopolymer, geopolymerization, Portland cement
In mineralogy, hardness is the resistance to scratching. Mohs’ scale of hardness is generally used to obtain relative hardness information by comparing (i.e., by a scratch test) an unknown to one of ten minerals defined in the Mohs’ hardness scale. Hardness tests are performed on a smooth surface of the unknown. Hardness is not an isotropic property and thus, may depend on the direction in which the surface is scratched, although the differences in most minerals are small.
Cf., Mohs hardness, Vickers hardness
An obsolete varietal term for biotite.
Hausmannite, Mn304, is tetragonal and has a deformed cubic spinel structure. Each Mn cation (Jarosch, 1987) in octahedral coordination is affected by Jahn-Teller distortions with two long Mn-O distances and four short distances. The Mn-O4 tetrahedra have equal Mn-O bond lengths. Hausmannite is isostructural with the Fe3O4 inverse spinel structure (magnetite). Hausmannite occurs in metamorphic manganese deposits, for example at Langban, Sweden, and Postmasburg, South Africa.