Commonly, cleavage planes … (a) Orange amphibole blebs developing along cleavage planes of a clinopyroxene in a clinopyroxenite nodule. The more than 100 amphibole species all have perfect cleavage in two directions and a splintery fracture. Most apparent, in hand specimens, is that amphiboles form oblique cleavage planes (at around 120 degrees), whereas pyroxenes have cleavage angles of approximately 90 degrees. With 2 or more cleavage planes present, it is important to pay attention to the angle of the cleavage planes. The iridescent blue variation is known as Labradorite. The pyroxene and amphibole chains are parallel to the c-axis of the crystal and their planes of basal O atoms ... cleavage takes place diagonally through the crystal and does not rupture the Si-O chains. Click on image to … The pyroxene group can appear in similar environments and has physical characteristics similar to the amphiboles: the two mineral groups can be distinguished based on the angles between their cleavage planes; the amphiboles have cleavage planes with 56˚ and 124˚ and the pyroxenes have 87˚ and 93˚. To determine the angle of cleavage, look at the intersection of cleavage planes. Fig.5: Schematic projection of the monoclinic amphibole structure on a plane perpendicular to the c axis showing the M1, M2, M3, M4 and A sites and I-beam structure yielding the typical near 60°-120° cleavage. The principal members of pyroxene and amphibole groups are listed in Table 4. Amphiboles are also specifically less dense than the corresponding pyroxenes. All the amphiboles, except anthophyllite are, monoclinic, and all show the excellent prismatic cleavage on (110). A nonmetallic mineral that is grey, grey-green, white, or blue in color, has 2 perfect cleavage planes at 90° with one plane showing striations (a growth pattern that appears as thin grooves), and a hardness of 6.0. Hornblende is a field and classroom name used for a group of dark-colored amphibole minerals found in many types of ... and two directions of excellent cleavage that intersect at 124 and 56 degrees. The amphibole supergroup is a group of rock-forming silicates that, in the field, have a similar appearance. A mineral with four directions of cleavage is termed octahedral.