Origin of Optical" Variations in Grossular-Andradite Garnet
Total Page：16
File Type：pdf, Size：1020Kb
American Mineralogist, Volume 69, pages 328-338, 1984 Origin of optical" variations in grossular-andradite garnet Mrzuntro Axrzuxr Institute of Mineralogy,Petrology, and EconomicGeology Faculty of Science,Tohoku University,Sendai 980,Japan Abstract Optical textures in birefringent grossular-andraditegarnets were studied from the standpoint of crystal growth. Grossular from Eden Hill, vermont, shows one-to-one correlation between growth surface features consisting of well-defined pyramidal hillocks and the internal texture. Grossular from Fujikaramari, Japan, consists of growth sectors with fine lamellaewhich apparpntly correspondto grofih steps.Andradite from Kamihogi, Japan, is composed of many triangular sectors which have the character of so-called lineage structure. The three specimens show sector twins whose planes are parallel, normal, or inclined to the [001] aiis in a thin section parallel to the (110)growth plane. The texture can be interpreted by a general mechanismof formation of order-disorder growth sectors suggestedby Akizuki (1981a).That is, various degreesof Fe3+/Al ordering in the octahedral cation sites were produced on the side faces of surface steps during growth. Introduction origin of sectors and twinning, although Tak6uchi et al. (1982) Optically non-cubic garnets, showing birefringence, suggestedthat the twinning ofgarnets they usedfor have been reported by many mineralogists and petrolo- X-ray refinement was produced during a transformation gists since the last century. Brauns (lE9l) summarized from disorderedto ordered structure after crystal growth. optical studies of birefringent garnets with sectors, and Through studies of a number of minerals showing so- Meagher (1980)reviewed recent studies. Birefringence is called optical anomalies, such as adularia (Akizuki and especially common in grossular-andraditegarnets (gran- Sunagawa, 1978),topaz (Akizuki et al.,1979), analcime (Akizuki, dite garnet), though garnets near both the end-member 1981a),and chabazite(Akizuki, l98lb), it was compositions are isotropic. Some grandite garnets show found that the internal sectors observable between alternating isotropic and birefringent lamellae parallel to crossed nicols correspond to growth planes and further- the {ll0}t growth planes(e.g., Murad, 1976).Also, gran- more that fine textures correspond to the various vicinal dite garnets show optical growth sectors which corre- faces on the growth planes, on which various degreesof cation or anion ordering may occur during growth. Also, spond to the {110} faces with or without {2ll} faces. Birefringent grandite garnet was transformed irreversibly a general mechanism of formation of order-disorder growth into optically isotropic garnet by heating at 860.C by sectors was suggested by the present writer (Akizuki, Stoseand Glass (1938),though the birefringencewas still l98la). problems observablein some garnets after quenching from a tem- Two of optically "abnormal" garnets are perature of 1060"C(Ingerson and Barksdale, 1943'5. considered here: (l) why is the optical symmetry lower The origin ofbirefringence in garnetshas been various- than that usually shown by X-ray analysesand morpholo- gy, (2) ly attributed to residual strain in the structu.re(Chase and and what is the origin of the complicated textures Lefever, 1960;Kitamura and Komatsu, l97E) and order- that the "abnormal" garnets show without exception? ing of octahedral cations, Fe3+ and Al, thereby reducing The former question was explained in terms of Fe3*/Al (Tak€uchi the symmetry (Tak€uchi et al., 1982). Takduchi et al. ordering et al., 1982), while the latter has (1982) refined the structures of sorne grandite garnets remained an open question since the last century. with X-ray data and found cation or{ering. In grandite The objective of the present paper is to describe the growth garnetfrom Munam, North Korea, the cont€nt of Fe3+in sectors and optical properties of some grandite garnets the eight non-equivalentoctahedral positions varied from showing distinct birefringence and to discuss the l5(l)% to 49(l)%, and the spacegroup was 1T. origin of the optical sectors and twinping using the general Winchell and Winchell (1951)summarized the sectoral mechanism suggestedby Akizuki (19E1a). twinning in birefringent garnets, though fine twinning in each sector was not described. Littte is known about the Observations Although birefringent grandite garnets from several I Throughout this paper indices refer to the ideal iscimetricaxes. localities were studied, only the garnetsfrom three local- m03-fix)v84l03M-032E$02.00 32E AKIZUKI: OPTICAL VARIATION IN GROSSULAR.ANDRADITE 329 ities are described in this paper. Most observationswere feature varies during different stages of crystal growth, madeon thin sections(30 g.mthick) parallel and normal to and therefore the correlation between optical properties the growth surface(110). and surface topography is not always good in some minerals(see Akizuki, 1981a).However, from the present is clear that the internal textures were Grossular from Eden Hill, Vermont observations, it producedduring growth. The garnethas cinnamon color and transparentdodeca- The optical orientation and 2V value in the light- hedralform {ll0}, which is the so-calledhessonite type, colored sectors of Figure lb are shown in Figure 2. The crystals are as large as 5 mm in diameter, and chemical optical vibration direction X is normal to the growth plane composition is Grs57 And1a.3.The surface microtopo- (110), and the other two directions, I and Z, rotate tp to graphy of crystal faces was studied with reflection-type 4ofrom [001] and [Tl0] around the X-axis. The 2V' value interferencecontrast microscopy. The (110)growth sur- face is characterizedby growth hillocks elongatedparallel to the [001] direction (Fig. la). Each hillock consists of four vicinal faces in a pyramidal form. The faces intersect in edges parallel to [001] and at about 50' to [001] (in projection perpendicular to (110)). Some indistinct boundaries, across which optical contrast differs, shown by white lines at the lowerleft on Figure la; Summits of growth hillocks are observed at the i tions of the boundaries. There are six summits bet two arrows marked A and A'. Also. the summit of a hillock with some growth layers in the central area irili Figure la is shown by arrow B. ,l A thin section parallel to the growth surface prepared from the specimen, whose as-grown face cementedon a glass slide, and was observed on thd'. ! polarizing microscope with a universal stage in order to :; 1; '.: measurethe optical orientation. Figure lb shows a photo-i' ; . " ...^^--... "'. summits.Six centersof fourlingsectors between A and' -/ A' of Figure 1b correspond to the six summits of growt hillocks shown in Figure la. Likewise, the center shown by arrow B in Figure lb correspondsto the summit of the growth hillock B in Figure la. Although the fourling vicinal faces cannot be observed clearly on the photo- graph (Fig. 1a),because of the small diference of inclina- tion between two of the vicinal faces, the fourling sectorsi are clearly observedbetween crossedpolars (Fig. lb).; The indistinct boundaries shown by the white lines at the lower-left in Figure la correspond to sector boundaries inclined about 50'to [001] in Figure lb. The angleof these boundaries varies from 90oto about 50'with respect to il ttre [001] axis from specimen to specimen. The growth hillock with summit B developedat a late stageof crystal i: growth, and one of the vicinal faces covered part of the growth hillock with summit indicated by a letter C (Fig. la). The sector boundary under the vicinal face is shown by a broken line. The growth hillock at C is much thicker cross polarized transmission (b) photomicrographs of a (ll0) as compared with the vicinal face covering the hillock C, section of garnet from Eden Hill. [001] is vertical, and the and therefore the sector boundary corresponding to the polarizing plane is rotated 3'clockwise from vertical. Seetext for broken line is distinct in Figure lb. Frequently, a growth details. 330 AKIZUKI : OPTICAL VARIATION IN GROSSULAR-ANDRADITE lar sectors, shown by arrows, correspond to one of the fourling sectors produced on the four vicinal faces of Eden Hill garnet (Fig. lb). The sectorsshown by stars have rims parallel to the outline of the (ll0) plane. A magnifiedschematic sketch of the texture in the rectangle of Figure 4 is shown in Figure 5. The extinction directions shown by arrows are inclined to [001] in the (110)thin section, though they are nearly parallel to [001] in some areas. Lamellae with different extinction angles repeat alternatelyparallel to one ofthe outlinesof{llOi facesin each sector. Thus, a clear correlation exists between orientationsof the lamellaeand extinction (Fig. 5). One of the growth sectors shown in Figure 5 has a rim with two extinction anglesinclined 15'to the (ll0) outline. This suggeststhat a thick hillock with large faces existed on the surface during growth. The fourling sectoral texture as found in Figure lb was not observed in this specimen. Figures 6a and 6b show the sectoral textures of another Fig. 2. Stereogaphicprojection of the optical properties measuredon a (ll0) thin sectionof garnetfrom Eden Hill. Principalvibration direction (X) is normalto the (ll0) growth surfaceand optic axesare indicated by OA. is (+)80'. The four sectors corresponding to the four vicinal faces on the growth hillocks are reflection twins to each other. Other transparentgrandite garnetsfrom Tori- no, Italy, showed a texture similar to that of Eden Hill gamet. However, detailed study was impossible, because of the small size of the crystals. G r o ss ul ar fr o m F uj ika r ama r i, F uku shi ma P r efec - ture, Japan, The specimens, which are red-brownish and translu- cent, have well-developed{ll0} faces with small {2ll} faces and are about l0 mm in diameter. The {110} faces have someirregular growth stepsand the {21l} faces have fine striations. Fine cracks, some of which are filled by calcite, are abundantin the crystal. Growth bandsparallel to the (ll0) face are due to chemical variations, which were observedwith an electron microprobe.